NATURE 



289 



THURSDAY, JULY 30, 1903. 



A MODERN PHYSICIST. 

 Scientific Papers of Lord Rayleign. Vols, ii., iii. 

 [901. Pp. xiv + 598; xii + 596; xiv + 



and iv., 1881- 



604. (Cambridge : University Press.) 



TO review these volumes in an ordinary sense is 

 an impossible task. Fortunately it is quite un- 

 necessary. Lord Rayleigh's work in its many phases 

 is so well known that a brief notification of the fact 

 that his papers have been collected and published by 

 the Cambridge University Press is almost all that is 

 called for. Every physicist will realise that his 

 library is incomplete without these four splendid 

 volumes, the first of which has already been noticed, 

 and that he will find in their pages the details of many 

 of the most striking advances in his subject during 

 the past twenty years. 



Lord Rayleigh succeeded Maxwell as professor of 

 physics at Cambridge in 1879. The first volume under 

 notice opens with his classical work in the Cavendish 

 Laboratory on the electric units ; the latter pages of 

 vol. iv. deal with his experimental verification of 

 Boyle's law for pressures down to the hundredth of a 

 millimetre. A list of the papers — 272 in number in 

 the four volumes — would cover the whole range of 

 physics, and each contains a contribution of real value 

 to natural knowledge. 



During his tenure of the Cambridge chair. Lord 

 Rayleigh undertook the determination of the three 

 fundamental units of electrical science, the ohm, the 

 ampere, and the volt, 



" It is generally felt," he writes in the first paper 

 {Proc. Roy. Soc, 1881), " that considerable uncertainty 

 still attaches to the real value of the ohm or British 

 Association unit of resistance. The ohm was con- 

 structed to represent 10' C.G.S. units of resistance, 

 but according to Kohlrausch it is nearly 2 per cent, 

 too great, and according to Rowland nearly i per 

 cent, too small." 



The ohm, {hanks to the work of Lord Rayleigh 

 and those who have followed in his steps, is now known 

 to some few parts in ten thousand. 



It is much the same with the ampere and the volt ; 

 more recent work has shown that possibly some small 

 change is required in the numbers given by Lord 

 Rayleigh to represent the electrochemical equivalent 

 of silver and the electromotive force of a Clark cell, 

 but the change will be very small. His work made the 

 Clark cell a practical standard, and every electrician 

 now knows its value. The H form of cell is first de- 

 scribed on p. 315 of the second volume of the papers. 



But this series of papers did not exhaust his experi- 

 mental work at Cambridge ; the researches on the value 

 of the ohm would have been incomplete without the 

 determination of the specific resistance of mercury 

 (Article 81) by Mrs. Sidgwick and himself. The ex- 

 periments on the rotation of the plane of polarisation 

 of light in a magnetic field were planned at first in 

 the hope of utilising the results in the measurement 

 of an electric current, and though this hope was not 

 realised, they remain as the standard determination 

 NO. 1761, VOL. 68] 



of Verdet's constant. A second paper on the Clark 

 cell is dated 1886. 



A short paper (No. 92) from the Philosophical 

 Magazine, vol. xiv., 1882, will serve as an example of 

 Lord Rayleigh's work as a critic. It is a comparison 

 of the methods for the determination of resistances in 

 absolute measure, and affords a most valuable risumi 

 of the methods employed. 



Resistance being on the electromagnetic system of 

 the dimensions of a velocity, the measurement of a 

 length and a time are necessary; the principal length 

 involved is nearly always the mean radius of a coil, 

 and the presumption is in favour of the method which 

 involves only a single linear measurement. 



The paper exhibits in a marked degree Lord Ray- 

 leigh's great capacity for seeing distinctly the essential 

 point of an experiment or a measurement, and keeping 

 that clearly in view throughout. This, indeed, is the 

 distinguishing feature of his experimental work, a 

 main factor in his success. Those who knew the 

 Cavendish Laboratory when the electrical measure- 

 ments were going on, or have since visited the labora- 

 tory at Terling, from which no less important work is 

 continually being published, have sometimes been sur- 

 prised at the makeshift character of much of the 

 apparatus. Contrivances of wood and wire and wax 

 do duty where most men would use apparatus elabor- 

 ated with a quite unnecessary care; but in Lord Ray- 

 leigh's case, while the essential instrument on which 

 the accuracy of the result really depends is as perfect 

 as the skill of the workman can make it, and, in 

 addition, has been thought out in all its details so 

 as to fit it best for the purpose immediately in view, 

 for the rest the arrangement which comes first to 

 hand is utilised without regard to appearances. 



In addition to the fundamental measurements 

 already referred to, the Cambridge years were marked 

 by a series of optical papers of great value. Among 

 these may be mentioned the article on optics for the 

 ninth edition of the " Encyclopaedia Britannica," in 

 which the theory of the resolving power of an optical 

 instrument is given in a simple manner. 



The papers already mentioned are contained in vol. 

 ii. of the collected works. Those in vol. iii., written 

 after Lord Rayleigh had resigned the Cambridge 

 chair, differ somewhat in character, but are no less 

 interesting. The article on the wave theory of light 

 from the " Encyclopaedia Britannica," and' the papers 

 1 the relative densities of hydrogen and oxygen, and 

 the composition of water. Articles 146, 153, 187, are 

 perhaps the most important. 



Attention may also be directed to a series of papers 

 on capillary questions, while Article 191, on the physics 

 of media that are composed of free and perfectly elastic 

 molecules in a state of motion, has a special interest. 

 Waterston had communicated to the Royal Society in 

 1845 a paper with the above title, which remained un- 

 published until 1892, when Lord Rayleigh's attention 

 was directed to it, and the paper was printed in the Phil. 

 Trans, with an introduction by himself. Waterston 

 was the first to introduce into the kinetic theory the 

 notion that heat and temperature are to be measured 

 b/ the kinetic energy of the moving particles. From 



O 



