360 



SCIENCE. 



[N. S,. Vol. XXIV. No. 612. 



the prophecy to which I have referred is 

 closely fulfilled. We may say, therefore, 

 with some confidence that the values of 

 those units which form the basis of our 

 system of electrical measurement are not 

 only practically determined with a high 

 degree of accuracy, but that also our meas- 

 urements of temperature and of energy are 

 placed on a satisfactory footing. 



The last few years have been fruitful in 

 revelations which not only profoundly 

 affect the views of students of science, but 

 also are of such a nature as to catch the 

 eye of the public. In some cases the ap- 

 plications of these discoveries to the pur- 

 poses of mankind have been evident and 

 immediate. Every well-equipped hospital 

 possesses apparatus for the production of 

 Rontgen rays, and I suppose that every 

 bluejacket in the navy has some degree of 

 acquaintance with those applications of 

 science which have resulted from the dis- 

 covery of Hertzian waves. 



The ambition of the student is naturally 

 fired by such examples, and there is a pos- 

 sible danger that the plodding but abso- 

 lutely necessary work of accurate measure- 

 ment may suffer by neglect. I, therefore, 

 venture to repeat the well-established axiom 

 that our advance in scientific knowledge is 

 a function of accurate measurement, and 

 that the student who devotes his energy to 

 the determination of some physical con- 

 stant is probably giving a 'point of de- 

 parture' to the pioneer. For it must ever 

 be remembered that to "the scientific investi- 

 gator the rule of three has ceased to hold 

 any significance. 



When Lord Rayleigh discovered that the 

 mean weight per liter under standard con- 

 ditions of chemical nitrogen was 1.251, and 

 that of atmospheric nitrogen was 1.257, the 

 believer in the rule of three would have 

 been unlikely to suspect that this difference 

 of 0.006 would supply the clue which led 



Lord Rayleigh and Sir W. Ramsay to the 

 discovery of a new element, a discovery 

 which in its turn led to others of possibly 

 even greater importance. For all we know 

 the next decimal place in any hitherto ac- 

 cepted value may afford another example 

 of the truth of the statement that a part 

 may be greater than the whole. 



At the time when Lord Kelvin delivered 

 the address to which I have already re- 

 ferred, the truth of the second law of 

 thermodynamics was probably not so gen- 

 erally accepted as is the case at the present 

 time. Each apparent example of violation 

 of that law has on closer examination 

 proved to be additional evidence of its 

 validity. We seem unable to find those 

 'sorting demons' of Maxwell's, the exist- 

 ence of which appears necessary for its 

 violation. 



Mr, Campbell recently expressed doubts 

 as to the application of thermodynamic 

 considerations to osmotics. He contended 

 that the errors in the determination of 

 osmotic pressure were greater than those 

 which could be attributed to experimental 

 sources. Now, the theoretical relation be- 

 tween osmotic pressure and the freezing- 

 point is based directly on thermodynamic 

 considerations, and it was because I enter- 

 tained a belief that the most direct evi- 

 dence of this much-debated matter could 

 be obtained from the observation of the 

 freezing-point of a very dilute solution 

 that I embarked on a series of somewhat 

 elaborate experiments during the years 

 1897 to 1901. My removal from Cam- 

 bridge and the death of my assistant, Mr. 

 C. Green, compelled me to leave that in- 

 quiry in an unfinished condition. Never- 

 theless, I had investigated the depression 

 of the freezing-point in certain solutions 

 varying in strength from 0.0003 to 0.025 

 gm. -molecule per liter. 



Subsequently to my departure from 



