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129 
fifteen years on the subject has been based on Maxwell’s 
fundamental equations, and is largely a result of his 
theoretical views. 
On Maxwell’s death (1879) Lord Rayleigh was 
appointed, and held the chair \intil 1884, when he re- 
signed to take the place in the Royal Institution 
vacated by the retirement of Tyndall. His short 
tenure of the Cambridge chair was marked by a series 
of classical researches in the Cavendish Laboratory on 
the value of the electrical units. Lord Rayleigh under- 
took a determination of the three fundamental units, 
the ohm, the volt, and the ampere, and performed this 
worl with an accuracy that has left little room for 
improvement. It is hardly necessary to speak here 
of his valuable work in this connection, which is so 
well known to every physicist, but it suffices to recall 
his experiments on the ohm with a modified form of 
the British Association revolving coil, his determin- 
ation of the electrochemical equivalent of silver and 
the E.M.F. of the Clark cell by means of his current 
balance, and his determination with Mrs. Sidgwick 
of the specific resistance of mercury. At the same 
time he determined in absolute measure the rotation 
of the plane of polarised light of carbon bisulphide in 
a magnetic field. In addition to this electrical work, a 
number of optical papers of great value were written 
within this period. We have confined our attention 
to the work of Lord Rayleigh in the Cavendish Labor- 
atory. To the great mass of valuable work produced 
before and after his stay in Cambridge (now collected 
and published in four large volumes) it is impossible 
even to refer in this short article. 
On the resignation of Lord Rayleigh, J. J. Thomson 
was appointed, at the early age of twenty-six. Like 
his predecessors in the chair, Prof. Thomson is a pro- 
duct of the mathematical and physical school of Cam- 
bridge, first taking the mathematical tripos and then 
entering upon experimental work in the Cavendish 
Laboratory. His first piece of work, undertaken before 
his appointment, was a determination of ‘‘ v ’’—that | 
important ratio between the electromagnetic and 
electrostatic units to which so much attention was de- | 
voted before the verification of Maxwell’s electro- 
magnetic theory. This was followed by a notable 
piece of mathematical analysis dealing with the action 
of vortex rings on one another, which gained for him 
the Adams prize. In this paper he investigated with 
great mathematical power the stability of interlocked 
vortex rings, and showed that not more than seven 
could be linked together without breaking up into new 
arrangements—a result which probably indicates the 
reason why no element has a greater valency than 
seven. In this work we have the first evidence of the 
bent of J. J. Thomson’s mind towards the study of 
the constitution of matter—a study to which he has 
devoted so much attention with such conspicuous 
success in recent years. Next followed the publication 
of a book on the application of dynamics to physics 
and chemistry—a notable work in which a general 
method of analysis, based on Lagrange’s equations, 
was used to solve many recondite physical and chemical 
problems. Among these may be mentioned an investi- 
gation of the action of an electrified atom in causing 
the condensation of water vapour around it. This 
result has proved to be of great importance in con- 
nection with later work to be done in the laboratory. 
The year 1887 saw the publication of a paper on 
the effect of a moving electrified sphere, not only re- 
markable for the direct results obtained, but for its 
indirect bearing on the question of the origin of mass. 
The results of a mathematical analysis showed that a 
moving charge of electricity possessed an apparent or 
electrical mass in virtue of its motion. This electrical 
mass was constant for slow speeds, but increased with 
great rapidity as the speed of light was approached 
NO. 1780, VOL. 69] 
until, at the velocity of light, it became infinite in 
value. The possibility that mass, which has been such 
a mystery to science, is due to electricity in motion 
has been recently brought much into evidence by the 
experiments of Kaufmann on the kathodic rays of 
radium. He has shown that the apparent mass of the 
particles constituting the kathode rays, spontaneously 
emitted by radium, increased with the speed in accord- 
ance with the theory first advanced by J. J. Thomson, 
and afterwards developed by Heaviside, Searle and 
Abraham. This result points to the possibility that 
the apparent mass of the kathodic ray. particle may be 
accounted for by electricity in motion without the 
necessity of any material nucleus. 
The following years were occupied partly with 
investigations on the electrodeless discharge, the 
electrification produced by falling drops of water and 
experiments on electrical oscillations, and also with the 
preparation for the press of a text-book on electricity 
and magnetism, and a_ splendid volume entitled 
“ Recent Researches in Electricity and Magnetism.’’ 
These two books are so well known to every physicist 
that no further mention is necessary here. 
J. J. Thomson next definitely attacked the problem 
of the nature of the discharge of electricity through 
gases. <A repetition of Perrot’s experiments on the 
passage of electricity through steam and experiments 
in vacuum tubes led him to the view that, as in a 
solution, the passage of electricity through gases was 
accompanied by electrolysis. This theory has been 
modified with the growth of experimental knowledge 
to the view that the discharge in gases is due to the 
motion of charged carriers or ions. These ions are 
not necessarily identical with the corresponding ions 
in the electrolysis of solutions. There is no doubt that 
there is in many cases an actual electrolysis similar 
to solutions occurring in gases, but this seems to be 
the result of a secondary action. 
A great impetus was given to the study of this 
subject by the discovery of Roéntgen rays. © These rays 
possess the power of making all gases temporary con- 
ductors of electricity. In a paper with Rutherford, 
J. J. Thomson advanced the view that the conductivity 
imparted to the gas by the rays was due to the pro- 
duction of positively and negatively charged ions in the 
gas. These ions travel in an electric field with a 
velocity proportional to the strength of the field. 
When no electric field is acting the ions gradually dis- 
appear by recombination amongst themselves. This 
theory was found to explain all the characteristic 
properties of the conducting gas. In the course of the 
next few years, as a result of the joint efforts of those 
engaged in research in the Cavendish Laboratory— 
among whom may be mentioned C. T. R. Wilson, 
Maclelland, Rutherford, Zeleny, Townsend, Langevin, 
H. A. Wilson, Maclennan and Strutt, and many more 
besides—the subject developed with great rapidity 
along two distinct lines. By purely electrical methods 
the ionisation theory of gases was shown to account 
for the conductivity of flames and vapours, the dis- 
charge due to ultra-violet light and to radio-active 
substances. At the same time the admirable experi- 
ments of C. T. R. Wilson on the detection of ions by 
means of their power of becoming centres for the con- 
densation of water vapour upon them showed that 
charged ions actually did exist distributed throughout 
the gas, and were not a figment of the imagination. 
During this time J. J. Thomson published a remark- 
able paper on the nature of the kathode rays. Since 
their discovery by Crookes, the nature of these rays 
had been the subject of what may almost be called an 
international controversy. The English school took 
the view that they consisted of a stream of matter 
projected with great velocity; the German school re- 
| . . . 
| garded them as a kind of wave motion in the ether. 
