THECavendish Professorship at the University of Cambridge, England, of which Sir J. J. Thomson is the present holder, is a post of honor to which his predecessors have lent no ordinary luster. The first holder of the office was James Clerk Maxwell, the profound original genius who laid the foundations for the modern electro-magnetic theory associated with his name. The second Cavendish Professor was Lord Rayleigh, whose measurements of the fundamental quantities of physics are among the most beautiful and accurate experiments ever performed. The third to be called to the chair ,vas Sir J. J. Thomson, who has proved himself in every way worthy of his predecessors. He was appointed while a young man in his twenty-seventh year—a very early age for so responsible a position—and his appointment was severely criticised by some of the' older and more orthodox professors and tutors. One well-known college tutor expressed the opinion that things had come to a pretty pass in the University when mere boys were made professors. But the Board of Electors, including Sir William Thomson (Lord Kelvin), Prof. G. G. Stokes, and Prof. G. H. Darwin, knew what it was doing, and the bold appointment has been amply justified by the continued advance of the Cavendish Laboratory and by the position which Sir Joseph has attained among his contemporaries. His early training Sir J. J. Thomson received at Owen's College, Manchester, where he was a student in 1876, and carried out some experimental work under Prof. Bal-four Stewart. He thence proceeded to Cambridge, where he read mathematics until taking his tripos in 1880. From this time dates his connection with the Cavendish Laboratory, which ,^as later to become so intimate and prolonged. He was appointed to his present position in 1884. Perhaps when the lapse of time shall enable us to see things in a truer perspective, we shall consider that his greatest service to science has been the building up and inspiring of the center of research at the Cavendish Laboratory. When he was first appointed there were not more than seven or eight students engaged in research, while at the present time the number has risen to about thirty, and the mass of work emanating from this scientific center is a splendid testimony to the enthusiasm and ability of the men gathered there. To give an account of any great man's work within the limits of a short article is always difficult, but it is unusually so in the present case, for Sir J. J. Thomson is so continually overflowing with ideas, that his contributions to science are extremely varied and numerous. It must suffice here to mention only a few of the larger pieces of work which he has carried out. In 1881 appeared as the Adams Prize Essay a mathematical treatise on the vortex theory. At that time the theory that atoms were vortices in the ether was very popular and several well-known names were connected with it; it had \ery little to recommend it, however, and since a better understanding of the phenomenon of the discharge of electricity through gases has given us more intimate information about the atom, the vortex theory has lapsed into a well deserved obscurity. Nevertheless the treatise formed a most valuable contribution to the general theory of vortices and showed some of the serious difficulties which would have to be overcome before the vortex theory of the atom could be accepted. Appropriately enough, one of Sir J, J. Thomson's earliest experiments was the repetition of a measurement which strongly supported Maxwell's electro-magnetic theory, namely, the determination of the ratio of the electro-static to the electro-magnetic unit of electricity. This was found to be equal to 3 X 10'°, a figure closely agreeing with the measured velocity of light expressed in centimeters per second. This result is one of the conclusions which Maxwell derived from his theory, which thus received strong corroborative evidence. Further confirmation was furnished in 1887 by the brilliant work of Hertz in demonstrating the existence of the electro-magnetic waves which Maxwell's theory foretold, and since then these waves have become a matter of every day talk by the development of wireless telegraphy. The next important work published by Sir J. J. Thomson was a series of papers published in the Philosophical Transactions' of the Royal Society in 1886-1887 (and republished in book form in 1888) on “The Application of Dynamics to Physics and Chemistry.” This work suggests a number of general methods of attacking chemical and physical problems which have proved most useful. Their greatest value lies in the fact that they can be applied even where very little is known of the inner processes going on. A very important experiment was carried out in 1889, when Sir J. J. Thomson devised a method of finding the specific inductive capacity of different substances for very rapidly alternating electric forces. This experiment was of considerable interest owing to the fact that Maxwell's calculations of the velocity of light in different substances, as dependent upon their SIR JOSEPH JOHN THOMSON specific inductive capacity, gave results considerably at variance with the observed values in certain cases. Sir J. J. Thomson suggested that the discrepancy might be due to the fact. that the specific inductive capacity of a substance for rapidly alternating electric forces, such as occur in light waves, differed from that found for steady electric forces in the usual way. Experiment completely bore out this suggestion, and added one more link to the chain of evidence in support of Maxwell's theory, After this followed a long series of experiments on the conduction of electricity through gases, which, along with the work of Schuster and others, has gone far to clear up our understanding of the mechanism of this complicated' phenomenon. In 1898 these experiments culminated in the discovery of the “electron,” or as Sir J. J. Thomson himself calls it, the “corpuscle.” This discovery has ushered in a new era in science, and has opened out a vast realm for new work upon the development of which some of the great physicists and mathematicians of the day have spent their best efforts. It is unfortunately quite impossible within the scope of this article to give even a remote idea of the bearing of this work; only one or two points shall be mentioned with which the name of J. J. Thomson himself is associated. The picture of the atom which Sir Joseph would present to us is that of a large number of corpuscles, assembled together into a minute planetary system, in which the corpuscles play the role of planets, revolving in concentric rings. From considerations of the stability of such rings he, has suggested an explanation of the existence of series or groups of elements with similar properties, thereby taking the first step in lifting the veil from the mystery of Mendelileff's Periodic Table of the Elements. Furthermore, by calculating the period of vibration resulting from various distortions of such rings of corpuscles, 'he has also suggested an explanation for the existence of series of lines in the spectra of different elements. Since this conception of the atom was first put forward by Sir J. J. Thomson, it has been found that some elements are continually breaking down, ejecting corpuscles, and giving rise to other elements as transformation products. Had the old idea of the indivisible atom still remained, this wouid have presented an almost insoluble problem, but with the new view of the atom it becomes perfectly simple. Breaking down the atom is simply knocking off some of the outer rings of corpuscles, leaving the inner rings intact, and it is evident that such a process must be accompanied by the ejection of corpuscles unless the rings which are knocked off reform themselves into a new system with exactly the same number of corpuscles as are broken off the parent atom. Since that time Sir J. J. Thomson's main experimental and theoretical work has been concerned with the further working out of the problems presented by the conduction of electricity through gases, and with the constitution of the a tom, but space does not permit of the presentation of his work in further detail. As a teacher he exerts a most inspiring influence upon his students, making himself one with them, and making them feel that he and they are engaged upon a common quest. This sympathy witli his pupils is probably the secret of his power, but he has other qualities which are very valuable to his students. His happy way of working things up from first principles, and thus recalling the strong and weak points in any theory, is most suggestive, clears up a man's ideas, and is most excellent preparation for future research work. '1'0 observe his use of mathematics is an education in itself. Sir J. J. Thomson is never led away into those pretty mathematical bypaths so dear to the heart of the orthodox Cambridge mathematical physicist. He selects such mathematics as is required for his investigation and then marches straight ahead for his goal in a most convincing fashion. He is always the physicist first and the mathematician after. As leader of the research students at the Cavendish Laboratory Sir Joseph is the most approachable and helpful of men. Although his personality dominates the laboratory, yet there is splendid freedom from restraint and a wonderful encouragement to carry out research on any subject. His visits to any particular student may be somewhat spasmodic, but this is quite in keeping with some other little eccentricities which are well known to all his students and which no doubt lose nothing in the telling. At tea, which he provides every afternoon in his room for all the research students, J. J., as he is always called, is perhaps at his best. At this time all the gossip of the laboratory passes around and a story has to be a pretty tall one if he does not manage to cap it J. J.'s vigorous radical utterances are also very warmly discussed, and not infrequently, among the cosmopolitan collection of students, the political discussions become both interesting and very animated. In this article the endeavor has been simply to show Sir J. J. Thomson as a man of science and the head of a famous laboratory. Of the many honors showered upon him, culminating in the award of the Nobel prize in 1906, and the bestowal of Knighthood in 1908, and of his home life, nothing has been said; but the writer hopes that enough has been laid down to make the reader join him in the wish that J. J. may be long spared to extend the limits of our knowledge in that section of natural science which he has made peculiarly his own, and in which he has labored with such brilliant success, giving to the world the priceless gift of his genius, and earning imperishable fame.