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The Scientific Work of John A. McClelland: A Recently Discovered Manuscript

Abstract

John Alexander McClelland (1870–1920) was educated at Queen’s College Galway and the Cavendish Laboratory in Cambridge. He was Professor of Experimental Physics at University College Dublin from 1900 to 1920. He was best known for his pioneering work on the scattering of β rays and on the conductivity of gases and the mobility of ions. He established a research school on atmospheric aerosols that was continued by his successor, John James Nolan (1887–1952), which strongly influenced physics research in Ireland up to the present. A recently discovered manuscript of a commemorative address by Nolan in 1920, which is reproduced in Appendix I, is a unique contemporary summary of McClelland’s research and character, and is an important contribution to the history of experimental physics in Ireland.

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Notes

  1. Archive for Atmospheric Aerosols, James Hardiman Library Archives, National University of Ireland Galway, University Road, Galway, Ireland, website <www.library.nuigalway.ie>.

  2. The student who scored highest on the Mathematical Tripos examination was called the Senior Wrangler, the one who scored second highest the Second Wrangler, and so forth.

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  46. Thomson and McClelland, “Leakage through Dielectrics” (Appen. II, ref. M1).

  47. McClelland, “On the Conductivity of the Hot Gases” (Appen. II, ref. M5); McClelland, “On the Conductivity of Gases” (Appen. II, ref. M6); McClelland, “On the Action of Incandescent Metals” (Appen. II, ref. M7).

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  54. McClelland, “On the Emanation given off by Radium” (Appen. II, ref. M9).

  55. McClelland, “On Secondary Radiation” (Appen. II, ref. M12); McClelland, “On Secondary Radiation (Part II)” (Appen. II, ref. M13); McClelland, “Energy of Secondary Radiation” (Appen. II, ref. M14); McClelland and Hackett, “Secondary Radiation from Compounds” (Appen. II, ref. M15); McClelland and Hackett, “Absorption of β-Radium Rays” (Appen. II, ref. M16); McClelland, “Secondary β-rays” (Appen. II, ref. M17).

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  59. McClelland and Kennedy, “Large Ions in the Atmosphere” (Appen. II, ref. M20).

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  75. Ibid., p. 376.

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Acknowledgments

I gratefully acknowledge the assistance of Aileen Fyfe in preparing my paper, and of Roger H. Stuewer for his thoughtful and careful editorial work on it.

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Correspondence to Thomas O’Connor.

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Thomas O’Connor studied physics at University College Dublin and is a retired lecturer at the National University of Ireland Galway with research interests in atmospheric aerosols and history.

Appendices

Appendix I: The Scientific Work of the Late Professor M’Clelland, by John J. Nolan

[Editorial Note: The original version of Nolan’s address is hand-written in blue-black ink on lined foolscap paper (figure 11). Pages 1 and 2 of its 21 pages appear twice, in draft and in a fair copy in black ink; the latter has been used in the transcription below. The rest of the manuscript is a first draft with some corrections and additions in the margins; the former have been incorporated and the latter appear in curly brackets { } in the transcription. The abbreviations have been completed, but slips in grammar have been kept, as have the original punctuation and spelling, for example, “M’Clelland” instead of “McClelland” and “kathode” instead of “cathode.” Editorial interpolations appear in square brackets [ ], as do references to McClelland’s papers as listed in Appendix II. For words that are hard to decipher a question mark (?) has been inserted beside the most probable word. Nolan’s manuscript was preserved, along with some copies of M.Sc. theses and other reports from the McClelland era, in the papers of his brother, Patrick J. Nolan, and then found its way into the papers of his brother’s successor, J. Anthony Scott, at University College Dublin. It is now preserved in an archive on atmospheric aerosols in the James Hardiman Library at the National University of Ireland, Galway.]

Fig. 11
figure 11

The first page of J.J. Nolan’s manuscript. Credit: Photograph by the author

It has seemed to me, as his successor in the chair of Experimental Physics in this College [University College Dublin, UCD], that the Presidential Address to the Scientific Society of this year, could not be other than a tribute to the scientific work of the late Professor M’Clelland. From its foundation Prof. M’Clelland was President of this Society. But he was much more than a nominal head—he was a continual support to the Society and a continual influence in its work. He aimed at making this Society a vital and an active expression of the scientific work of the College and he strove to keep it continually in touch with the scientific work of the day. But even that was only a small part of his work on behalf of science in this College. This building in which we meet today, these laboratories and research-rooms which you have seen, are a permanent expression of the devotion which he has given to scientific research. The thousands of students whom he taught and the smaller numbers of these whose footsteps he guided on the paths of scientific investigation will hold his memory in life-long reverence.

As one of his pupils—and very unworthy of his place in this College to which I have succeeded—it is not without great diffidence that I undertake to deal with his scientific work and endeavour to pay some tribute to the additions to knowledge which were the fruit of the all-too-brief 25 years which he devoted to Science. I think however, that it is safe to say that appreciation of his scientific work, is perhaps the one tribute that he would have cared for.

Prof. M’Clelland was educated at the Academical Institute, Coleraine and at Queen’s College, Galway. In the year 1894 he went as a research student to Owens College Manchester {Schuster?} [Professor Arthur Schuster] where he remained for a year. In the following year he went to Cambridge, joining that remarkable group of men whom J.J. Thomson had gathered around him in the Cavendish Laboratory, and who have had so much to do with the making of modern physics.

It is well at this stage that we should try to get an idea of the state of experimental physics in the year 1895 when M’Clelland began his work, for we shall find that a survey of his work is to a large extent a survey of the modern developments of physics.

At that time the idea was held by some, that the great exploring work of physical science was over—that nature had yielded most of her great secrets that were accessible to experiment—that the work of the scientific man henceforth must lie in the accurate quantitative examination of the known phenomena, the accurate surveying of the known ground. Extraordinary short sighted as we would consider this view at the present day—still it was held and expressed by some scientific men. {Stagnation?} Yet—say in the [18]80’s—there was much development of great interest and importance. To take only two cases we have the remarkable work of Sir W[illiam] Crookes and others on electrical discharges in vacuum tubes and the experimental demonstration of electric waves by [Heinrich] Hertz. The Crookes tube was later to be the key to unlock a great many mysteries. But as has been pointed at by [Robert A.] Millikan the remarkable discovery of Hertz was to prove a hindrance rather than a help to the experimental physicist. For this reason. This brilliant experimental verification of the math[ematical] theory laid down many years before by [James] Clerk Maxwell naturally turned men’s minds to the ether rather than to matter. Electricity was something in the ether rather than something flowing along a wire. Now the ether is a singularly barren subject for experimental or indeed any other kind of research. How many years and how much effort did Lord Kelvin, greatest of the British physicists of his day, give in the search for a comprehensive theory of ether and matter in all their relations. Trying to picture matter as a state of the ether, he imagined atoms as vortices in the ether. Then we had elastic solid and all sorts of other ethers. Mendelieff even went so far as to attribute to it an atomic weight. Nowadays I’m afraid, the ether gets less respect from scientific men. I think one could almost tell the age of a scientific man from his attitude towards this once universally respected medium. As [Arthur S.] Eddington has pointed out recently we have ceased to endeavour to explain matter, by attributing the experimentally observed properties of matter to something which we assume to be non-matter. If (he says) physics evolves a theory of matter which explains some property it stultifies itself when it postulates that the same property exists ?—? in the primitive basis of matter {see Eddington}.

But just about the years 1894–[189]5 a remarkable series of discoveries were made which were to prove the beginning of a grand experimental attack. The attack has not ceased nor even noticeably diminished in vigour even to the present day.

Thus in Aug[ust] 1894 Lord Rayleigh announced the discovery of argon. In 1895 [Wilhelm C.] Roentgen discovered the X-rays. In 1896 [Henri] Bequerel discovered the radioactivity of uranium. In 1896 [Pieter] Zeeman discovered the effect of a magnetic field on the nature of light emitted by a radiating body. In 1897–1898 J.J. Thomson discovered the electron or atom of electricity in the discharge tube, measured its velocity, its mass and its charge. Each of these discoveries may be said to have opened up a new world for investigation. Now as I have said M’Clelland went to Cambridge in 1896. He has told how going to Cambridge he read in the train an account of the discovery of X-rays. And in Cambridge he found, working under the direction of Prof. Thomson, a group of men who were to be the pioneers of the new physics. With [Ernest] Rutherford, [John S.E] Townsend, C.T.R. Wilson, [Paul] Langevin and others for the next five years he took his part in the great movement. It was an inspiring time when almost every day brought a new discovery—and each discov[ery] of capital importance.

I will first deal therefore with this Cambridge period.

The first work undertaken by M’Clelland was the investigation of the effect of the newly discovered X-rays on gases. The results of this work were published in March 1896 [M1] and the paper is a remarkable one for in it we find practically all of the well known features of X-ray activity in gases dealt with. It was found of course that under the action of the X-rays the gas became electrically conducting. This conductivity was experimentally examined in an exhaustive way. The relative conductivity of diff[erent] gases was tested, and of the same gas at diff[erent] pressures, and at different temperatures. The important result was established that for this new kind of conductivity Ohm’s law was not obeyed. Then again, using the conductivity of the gas as a method of measuring the X-rays experiments were carried out to locate the exact source of the radiation. In the same way the absorbing powers of different materials for X-rays was tested. The very important result was established that X-rays are not homogeneous i.e. a given thickness of material did not always absorb the same fraction of X-rays that fell on it. [M2] Now at the time the method by which the conduction of electricity is carried on in a gas was not known. Shortly after it was shown that the current in a gas is conveyed by means of carriers or ions. These are minute bodies, of molecular size or thereabouts, carrying each a positive or negative charge of electricity. These are produced in the gas by the action of the X-rays and as was shown later by the radiation from radioactive bodies. The current is conveyed by the movement of these in the gas—the positively charged ones moving with the current and the negatively charged ones moving against it.

We see then that in his very first experiments M’Clelland was dealing with this question of gaseous ionisation to the knowledge of which he was later destined to contribute so much. And we see that, although at the time of his work the mechanism of the process was not known, still he elicited some of the cardinal features of the behaviour of ionised gases.

Now as so much of M’Clelland’s work deals with ionisation I think it would be well at this stage to indicate the interest and the importance of the study of gaseous ions.

An ion is a very small particle of matter—a molecule or a small cluster of molecules, carrying an electric charge. Generally speaking each ion carries an atomic charge of electricity. By the aid of its charge we can experiment on it, set it in motion: because of its charge we can recognise and follow and examine its motion. We can thus investigate the behaviour of matter in the minutest state of division under the simplest conditions. We are at the same time investigating the behaviour of the ultimate particles of electricity. We are not dealing with long-range phenomena; electricity or matter in the bulk, but with very intimate processes.

In the next year (1897) M’Clelland published two papers dealing with the character of the X-rays [M2] and with the Lenard and kathode rays. [M3] He extended his previous observations as to the absorption of X-rays by different substances and emphasises the non-homogeneous character of the X-rays. He showed that the non-homogeneity of the rays depended a good deal on the hardness of the tube. {Harder tubes give more non homogeneous rad[iation]} {Lenard Rays. Explain.} In connection with the Lenard rays he showed experimentally that they were of the same nature as the kathode stream and he also investigated the fraction of the current in a discharge tube carried by the kathode rays.

In the following year he published an interesting paper dealing with the effect of electric discharges on photographic plates. [M4] And then began his well-known researches on the ionization produced by flames, arcs and incandescent metals. This work is contained in three papers published between 1898 and 1902, [M5, M6, M7] but was completed before he left Cambridge in 1900.

From a very early time it was known that flames and flame gases had conducting properties for electricity. [William] Gilbert, [Alessandro] Volta and [Michael] Faraday all knew and made use of these properties. But while the phenomenon was well known and had been investigated by a multitude of experimenters during the nineteenth century, the nature of the action was not understood. M’Clelland’s investigations cleared up the matter and gave a complete theory to account for the mechanism of this conductivity.

M’Clelland showed that the conductivity of flame gases was due to the fact that they contained large numbers of ions, that is carriers of electricity, positively and negatively charged. He investigated the way in which the ionisation decayed by the combining together of positives and negatives to form neutral groups. {conductivity decreased}. He devised a method to measure the mobility of these ions that is to say the velocity with which they move under a unit electric field. This method is now one of the standard methods for investigating ionisation of any kind. He showed from the results of his measurements that the ions in the flame gases are larger than those produced in air or other gases by X-rays or radium, and he found that they increased in size with the lapse of time. This increase with time he suggested was due to the water vapour condensing around the ion. This is really the first discovery of what has since been called the large ion. We distinguish between the small ion, which appears to be either a single molecule or a small cluster of molecules and which is apparently a stable entity and the large ion which is a much larger group or cluster and which comes to a condition of stability or semi-stability under the operation of some different laws, not yet clearly understood. M’Clelland examined the ionisation produced by electric arcs and found that it was of the same character as that due to flames. [M6] He then made a very detailed examination of the ionisation produced by incandescent metals {thermions} and by working at low pressures was able to demonstrate the production of fresh ions by impact of ions already formed against neutral gas molecules. [M7] The whole question of the ionisation due to hot wires is now of the greatest practical importance. The modern use of thermionic valves in wireless work and in kindred technical processes which has led to such rapid and far reaching developments; owes much to M’Clelland’s pioneer investigations.

In the year 1900 M’Clelland was appointed to the Professorship of Physics in University College [Dublin]. The years that followed were for M’Clelland years of great scientific activity. All the time that was free from his lecturing duties was devoted to his experimental work which was carried on in the laboratory of the Royal University [of Ireland]. He began by an investigation ?with? the ionisation of atmospheric, that is the natural, ionisation always found in the atmosphere. [M8] He devised a method of measuring this ionisation and examined its variation under different conditions. He found that it tended to reach a maximum after rain fall and suggested that some part of it at least was due to radioactive matter brought down by the rain. Then he undertook an investigation into the emanation and the penetrating radiation from radium. He succeeded in showing experimentally that the emanation of radium had no electric charge. [M9] {A result of great importance from the point of view of the theory of radioactive transformation.} Then examining the penetrating radiation from radium, called the γ rays, he showed that unlike the α and the β rays they carried no electric charge. He examined the law of their absorption by matter and suggested, what has since come to be held as the true view, that the γ radiations are akin to X-rays, that is to say that they are a wave motion like light, not a stream of charged particles like α and β-rays. [M11]

The principal feature of M’Clelland’s work during this period was the great investigation which he carried out on the secondary radiation from substances exposed to the radiations from radium. [M12] When the rays from radium are allowed to fall on any substance the substance itself begins to emit a radiation, of pretty much the same character, but of course less intense than that which has fallen on it. [M13] M’Clelland examined this secondary radiation from different substances and found the remarkable result that the intensity of the secondary rays was closely connected with the atomic weight of the substance and further that the manner in which the secondary radiation varied with the atomic weight depended upon the position of the substances in the periodic classification of the elements. [M14] It appeared therefore that the secondary radiation was an intimate atomic effect. This was in fact further shown by a series of experiments on compounds in which M’Clelland demonstrated that the secondary effect for a compound was the sum of the effects due to its atoms. [M15] It is usual nowadays to give the term “scattered radiation” to what M’Clelland termed “secondary radiation”; but as M’Clelland points out in one of his papers “whether the expelled particles (that is the high velocity electron stream of which the radiation consists) are original constituents of the atom, or incident particles absorbed by the atom and subsequently expelled does not really amount to any essential difference.” Much work has been done on this subject by subsequent experimenters, but it cannot be said that they have added much to the results obtained by M’Clelland. In connection with this work investigations were also carried out on the mechanism of absorption of β rays by matter. A theory of this process was put forward by M’Clelland which has been the basis of further research into this difficult subject. [M16, M17]

Meanwhile M’Clelland’s work was winning for him wide spread recognition. In the year 1906 he became Secretary of the Royal Irish Academy and in 1909 he was elected to the Fellowship of the Royal Society. In 1908 he was nominated as member of the first Senate of the National University [of Ireland]. To the work of organising the newly established University and its Colleges he devoted much time and energy during these years. He was especially anxious as to facilities for research schools in science generally but especially in physics. In the old buildings of the Royal University [of Ireland] he soon had several of his students at work, and those who were with him in these days will remember the keenness of his spirit and the enthusiasm with which he infected all who were associated with him. Meanwhile the plans for the new buildings were being thought about and here again he was urgent and anxious as to provision for research. This building with its laboratories and research rooms which grew up under his hands are a record of the devotion which he gave to his work.

We come now to deal with his scientific work of this—the third period—the period since the establishment of the National University [of Ireland]. This work as we have said, was in part carried out in the old Royal University [of Ireland] buildings—and later in the present new building. While of a very varied character, for the most part it has some connection with ionisation and atmospheric electricity. This was partly due to the fact that in recent years the possibility of work on the radioactive substances did not exist. M’Clelland’s researches on radioactivity were carried out with some 50 mg [milligrams] of radium bromide which the Royal Dublin Society had purchased at his suggestion in the early days when radium was cheap. This radium he had on loan for a number of years but when the R[oyal] D[ublin] Society decided to establish a radium institute, for supplying radioactive preparations to medical practitioners the Society naturally called in the radium which it had lent to M’Clelland. One is tempted to enquire whether it is likely that these 50 mg of radium have done as much for medical science in the last few years as they did for physics in the hands of M’Clelland.

A cardinal problem in atmospheric electricity is the origin and maintenance of the electric charge on the earth’s surface and in this connection it is of importance to know whether rain or snow have any electric charge and if so how much and of what sign. In the year 1911 satisfactory experimental data existed only for one region and that in the tropics. For Europe the results were contradictory and confusing. The question also had a wider bearing in connection with the mechanism of condensation and the ionisation of the atmosphere. M’Clelland set out to work on this point and the results obtained here in Dublin have had considerable part in clearing up the question. [M18, M19] M’Clelland’s interest in this as in other problems of atmospheric electricity was sustained, for we find that in his last [sic] paper, he deals again with the electricity of rainfall. [M27] About the same time as the research on rain he initiated an enquiry into the nature of the larger ions and other nuclei in the atmosphere. This work had a close bearing on his earlier work in Cambridge on flame gases. Perhaps I may be permitted to state here broadly the results of this work. It had been known for a considerable time that when condensation takes place in the atmosphere—that is when a cloud forms—the water condenses into tiny globules each forming round a nucleus. If there is no nucleus, there is no condensation. As to [the] nature of this nucleus, it was generally called “dust.” Now what Prof. M’Clelland and Dr Kennedy showed was practically this: that these nuclei are not what we ordinarily call “dust.” [M20] They are particles of some sort most of them electrically charged, and all uniformly of the same size. They are produced by flames and fires principally and exist in many thousands per cubic inch of the air of cities. They are much less numerous in the pure air of the country. They must not be thought of as “dust.” They are too small and too regular—they are the large ions. They play a very important part in many natural phenomena and the whole question of their origin and behaviour is of the greatest interest.

Many other kinds of ions were investigated by M’Clelland and his pupils, ions produced by the spraying and bubbling of liquids [M23, M25] and ions produced by the combustion of phosphorus. [M26] In all these processes it would seem that tiny fragments of matter are detached and acquire an electric charge in some way. The electric charge, as I pointed out already, enables us to manipulate them and to study their behaviour. Much knowledge has been gained in recent years by M’Clelland and his pupils of the behaviour of matter in these highly divided forms.

Other researches in this period deal with the photo-electric effect in leaves, [M22] the conductivity of substances in thin layers, [M21] the conductivity of liquid dielectrics and the ignition of ether–air mixtures. [M28] But I would like to mention especially a research carried out on frictional electricity. This is the oldest in a sense the fundamental and certainly the least understood effect in electricity had always presented enormous difficulties to the experimenter. It was difficult to obtain any experimental result which could be repeated with certainty. There was no firm ground anywhere, everything was baffling and disconcerting. To this difficult subject M’Clelland was able to make some valuable contributions. [M24] By a series of well planned experiments he succeeded in establishing certain relations between the electric charge produced by friction and the temperature, humidity and gaseous pressure of the medium. We cannot say that much is yet known about the nature of this phenomenon, but at least as the result of M’Clelland’s work certain paths have been opened up which are bound to yield greater knowledge to future explorers.

We have now passed in review the contributions to scientific knowledge made by Prof. M’Clelland. We have dealt briefly with the fruitful work which naturally divided itself into three periods—the Cambridge period, the Royal University [of Ireland] period and the last period which ended prematurely almost before his real work of building up a research school was properly begun. But in a review of his scientific work we should not consider him altogether as a research worker and as a leader in research; we should consider him also as a teacher. And as a teacher his gifts were very great; perhaps it was the same gift that made him great in both departments—a remarkable clarity of mind, a power of cutting away the unessential and the accidental and getting at the realities of phenomena. He was equally great as a lecturer to elementary and advanced classes. The many students who attended his elementary lectures will remember the clearness and simplicity of his exposition. It was characteristic of him that he preferred that in his elementary classes there should be no taking of notes; the understanding by the student of the thing itself was what he aimed at—not the imparting of formulae.

As a lecturer to advanced classes he was unequalled. Here his great powers of clear thinking, accurate reasoning and plain exposition were fully revealed. In the hands of some men nature is obscured by a mist of mathematical analysis: M’Clelland was always insistent on the physical realities. I do not mean that he rejected the fullest applications of mathematical reasoning—quite the contrary—but he was insistent throughout in the recognition of the physical meaning behind the mathematical symbol. He did not perplex his students by unexplained assumptions, sudden jumps of thought or a too facile treatment of real difficulties—everything was reasoned out fully, everything treated from the point of view of the explorer. He aimed at giving his students the research point of view and he liked to set them at an early stage at some original or quasi-original investigation. His point of view was that of the student, who has acquired the necessary fundamental knowledge and the necessary initiation into experimental methods, his own gain will be as great and the gain of knowledge in general will be greater if he works out something which is new rather than something merely routine.

From the consideration of M’Clelland as a teacher we are thus led back again to his aspect as a leader in research. And naturally so for the idea of research dominated all his teaching. M’Clelland was too great a teacher and devoted too much care and energy to the teaching side of his work for there to be any risk of misunderstanding if I say that he always held teaching to be secondary to research. Indeed I think it would be still more correct to say that he made research the first interest of his life.

It is interesting to note certain points about his direction of the research work of students. He much preferred to set a student making observations and measurements at once rather than let him spend much time in conning the results of previous workers. The time for that would come later—but he wished the experimental attack and the ideas that developed in connection with it to be fresh and uninfluenced by what had gone before. Then again he strenuously steered away students from wandering off on any of the inviting side issues that invariably present themselves in any piece of work. He expected from a student hard work and patience—just as he himself would give both to the solution of any problem and while never in any way damping the enthusiasm of youthful workers—his dominant note with them as throughout his own work was caution.

At the beginning I said that the review of M’Clelland’s work is in a large degree a review of the newer physics of the last 25 years. We have dealt with his work on Kathode Rays, Lenard Rays and X-rays and then with his original studies in ionisation; later his researches into the radiations of radium. In recent years he had we may say founded a school of his own for the study of ionisation. But though in addition to that main interest many other enquiries were as I have shown, carried out with great success, still in practice, many lines of experiment were inaccessible. I have already shown how the loss of the radium closed one path: in the absence of a liquid air plant and of highly trained mechanical aid other lines of investigation are practically barred. But how many men with the most elaborate equipment have done as much for science as M’Clelland did with resources in some degree circumscribed? Indeed it was not altogether a disadvantage to him to have to contrive his experiments in a simple form. He had the same fondness for simplicity as say [George Gabriel] Stokes or Lord Rayleigh and something of the same powers of wresting from some simply contrived experiment important natural truths. If therefore he was precluded from any of the more “fashionable”—if I may use the word—types of research, perhaps the gain is all the greater. On the other hand good scientific work can not be done without reasonable facilities and the best work of any worker however able would be better done and done with far less toil if suitable resources were available. If the modern world is in any degree grateful for what science has given it or has any interest even the most utilitarian in the gaining of knowledge it should give its scientific men and indeed scholars of any sort a fairly free hand. All this is very close to the subject of my paper for M’Clelland’s zeal for the encouragement of research was well known and much of the time and energy of his later years was given to work in connection with the Committee for Scientific and Industrial Research.

It is given to few men to leave so permanent a record of themselves as M’Clelland has left—not merely in the science of his day but in the scholarship of his country. To the great Irish theoretical physicists of the last century [James] MacCullagh and [George Francis] Fitzgerald we may add his name as the first great experimental physicist. In the school which he created, as in the material fabric of laboratories which grew up under his inspiration he has made his place a permanency in this College and University. And in the minds of all associated with him as colleagues or as pupils he [his?] memory remains as that of a great leader and a true and kind friend.

Appendix II: The Scientific Papers of John Alexander McClelland (1870–1920)

M1.

J.J.Thomson and J.A. McClelland, “On the Leakage of Electricity through Dielectrics traversed by Röntgen Rays,” Proceedings of the Cambridge Philosophical Society 9 (1896), 126–140.

M2.

J.A. McClelland, “Selective Absorption of Röntgen Rays,” Proceedings of the Royal Society of London 60 (1897), 146–148.

M3.

J.A. McClelland, “Cathode and Lenard Rays,” Proc. Roy. Soc. Lon. 61 (1897), 227–235.

M4.

J.A. McClelland, “On the figures produced on photographic plates by electric discharges,” Proc. Camb. Phil. Soc. 9 (1898), 522–525.

M5.

J.A. McClelland, “On the Conductivity of the Hot Gases from Flames,” The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 46 (1898), 29–42.

M6.

J.A. McClelland, “On the Conductivity of Gases from an Arc and from Incandescent Metals,” Proc. Camb. Phil. Soc. 10 (1900), 241–257.

M7.

J.A. McClelland, “On the Action of Incandescent Metals in producing Electric Conductivity in Gases,” Proc. Camb. Phil. Soc. 11 (1902), 296–305.

M8.

J.A. McClelland, “Ionization in Atmospheric Air,” The Scientific Transactions of the Royal Dublin Society 8 (1903), 57–64.

M9.

J.A. McClelland, “On the Emanation given off by Radium,” Sci. Trans. Roy. Dub. Soc. 8 (1904), 89–94; “On the Emanation given off by Radium,” Phil. Mag. 7 (1904), 355–362.

M10.

J.A. McClelland, “The Comparison of Capacities in Electrical Work: An Application of Radio-Active Substances,” The Scientific Proceedings of the Royal Dublin Society 10 (1904), 167–177; “The Comparison of Capacities in Electrical Work; an Application of Radioactive Substances,” Phil. Mag. 7 (1904), 362–371.

M11.

J.A. McClelland, “The Penetrating Radium Rays,” Sci. Trans. Roy. Dub. Soc. 8 (1904), 99–108; “The Penetrating Radium Rays,” Phil Mag. 8 (1904), 67–77.

M12.

J.A. McClelland, “On Secondary Radiation,” Sci. Trans. Roy. Dub. Soc. 8 (1905), 169–182; “On Secondary Radiation,” Phil. Mag. 9 (1905), 230–243; “Secondary Radiation,” Nature 71 (1904 [February 23, 1905]), 390.

M13.

J.A. McClelland, “On Secondary Radiation (Part II), and Atomic Structure,” Sci. Trans. Roy. Dub. Soc. 9 (1905), 1–8.

M14.

J.A. McClelland, “The Energy of Secondary Radiation,” Sci. Trans. Roy. Dub. Soc. 9 (1906), 9–26.

M15.

J.A. McClelland and F.E. Hackett, “Secondary Radiation from Compounds,” Sci. Trans. Roy. Dub. Soc. 9 (1906), 27–36.

M16.

J.A. McClelland and F.E. Hackett, “The Absorption of β-Radium Rays by Matter,” Sci. Trans. Roy. Dub. Soc. 9 (1907), 37–50.

M17.

J.A. McClelland, “Secondary β-rays,” Proc. Roy. Soc. Lon. [A] 80 (1908), 501–515.

M18.

J.A. McClelland and J.J. Nolan, “The Electric Charge on Rain [Part I],” Proceedings of the Royal Irish Academy [A] 29 (1912), 81–91; “La charge électrique de la pluie,” Le Radium 9 (1912), 277–282.

M19.

J.A. McClelland and J.J. Nolan, “The Electric Charge on Rain (Part II),” Proc. Roy. Irish Acad. [A] 30 (1912), 61–71; “Sur la charge électrique de la pluie,” Le Radium 9 (1912), 421–426.

M20.

J.A. McClelland and H. Kennedy, “The Large Ions in the Atmosphere,” Proc. Roy. Irish Acad. [A] 30 (1912), 72–91; “Les gros ions dans la l’atmosphère,” Le Radium 10 (1913), 392–400.

M21.

J.A. McClelland and J.J. Dowling, “The Electrical Conductivity of Powders in Thin Layers,” Proc. Roy. Irish Acad. [A] 32 (1915), 51–58.

M22.

J.A. McClelland and Rev. R. Fitzgerald, “Photo-Electric Discharge from Leaves,” Proc. Roy. Irish Acad. [A] 33 (1916), 1–8.

M23.

J.A. McClelland and P.J. Nolan, “The Nature of the Ions Produced by Bubbling Air through Mercury,” Proc. Roy. Irish Acad. [A] 33 (1916), 24–34.

M24.

J.A. McClelland and Rev. C.J. Power, “Electrification by Friction,” Proc. Roy. Irish Acad. [A] 34 (1918), 40–50.

M25.

J.A. McClelland and P.J. Nolan, “The Ions produced by Bubbling Air through Alcohol,” Proc. Roy. Irish Acad. [A] 34 (1918), 51–61.

M26.

J.A. McClelland and P.J. Nolan, “The Nature of the Ions produced by Phosphorus,” Proc. Roy. Irish Acad. [A] 35 (1919), 1–12.

M27.

J.A. McClelland and A. Gilmore, “Further Observations of the Electric Charge on Rain,” Proc. Roy. Irish Acad. [A] 35 (1920), 13–29.

M28.

J.A. McClelland and Rev. H.V. Gill, “An Investigation into the Causes of the Self-Ignition of Ether-Air Mixtures,” Sci. Proc. Roy. Dub. Soc. 16 (1920), 109–119.

M29.

J.A. McClelland and J.J. M’Henry, “Uncharged Nuclei Produced in Moist Air by Ultra-Violet Light and other Sources,” Sci. Proc. Roy. Dub. Soc. 16 (1921), 282–303.

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O’Connor, T. The Scientific Work of John A. McClelland: A Recently Discovered Manuscript. Phys. Perspect. 12, 266–306 (2010). https://doi.org/10.1007/s00016-010-0023-8

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  • John A. McClelland
  • John J. Nolan
  • Patrick J. Nolan
  • Alexander Anderson
  • Arthur W. Conway
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  • Hugh Ryan
  • John A. Scott
  • J.J. Thomson
  • John S.E. Townsend
  • Queen’s College Galway
  • Queen’s College Cork
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  • Cavendish Laboratory
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  • Royal University of Ireland
  • National University of Ireland
  • Royal College of Science for Ireland
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  • beta-ray scattering
  • mobility of ions
  • atmospheric aerosols
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