PRESIDENTIAL ADDRESSi 47 



I* 



hospital possesses apparatus for the production of Rontgeu i-ays, dnd 1 suppose 

 that every bluejacket in the Navy has sonic degree; of acquaintauce with those 

 applications of science which have resulted from the discovery of Herzian waves. 



The ambition of the student is naturally fired by such examples, and there is a 

 possible danger that the plodding but absolutely necessary work of accurate 

 measurement 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 deter- 

 mination of some physical constant is probably giving a ' point of departure ' to 

 the pioneer. For it must ever be rememembered that to the scientific investigator 

 the rule of three has ceased to hold any significance. 



When Lord Rayleigh discovered that the mean weight per litre under standard 

 conditions of chemical nitrogen was 1'2.51, and that of atmospheric nitrogen was 

 r257, the believer in the rule of three would have been unlikely to suspect 

 that this difference of -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 tlie next 

 decimal place in any hitherto accepted 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 

 referred the truth of the second law of thermodynamics was probably not so 

 generally 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 whose 

 existence appears necessary for its violation. 



Mr. Campbell recently expressed doubts as to the application of thermo- 

 dynamic considerations to osmotics. He contended that the errors in the deter- 

 mination of osmotic pressure were greater than those which could be attributed to 

 experimental sources. Now, the theoretical relation between osmotic pressure 

 and the freezing-point is based directly on thermodynamic considerations, and it 

 was because I entertained a belief that the most direct evidence 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 experi- 

 ments during the years 1897 to 1901. My removal from Cambridge and the death 

 of my assistant, Mr. C. Green, compelled me to leave that inquiry in an unfinished 

 condition. Nevertheless, I had investigated the depression of the freezing-point in 

 certain solutions varying in strength from O'OOOS to 0'025 gm.-molecule per 

 litre. 



Subsequently to my departure from Cambridge Mr. Bedford re-erected the 

 apparatus in another building. After having surmounted great difliculties, he 

 repeated many of my experiments, and be informs me that the numbers he has so 

 far obtained are in almost entire agreement with those previously obtained by me. 

 The molecular depression in the case of cane sugar I found to be 1'858, of 

 potassium chloride 3-720, and I understand that Mr. Bedford's experiments agree 

 with these results with a discrepancy of less than 1 part in 1000. The most 

 probable number obtained from theoretical considerations would be in the former 

 case 1"857, in the latter 3'714. As Mr. Whetham has pointed out, unless there is 

 some balancing of opposite errors of a very improbable nature, it is difficult to 

 imagine a more direct vindication of the application of thermodynamic considera- 

 tions to the phenomena of solution. I may add that I also examined corre- 

 spondingly dilute solutions of sodium chloride, barium chloride, sulphuric acid, 

 potassium bichromate, magnesium chloride, and potassium iodide ; but, owing to 

 the circumstances to whicJi I have referred, I was unable to repeat these experi- 

 ments in such a manner as to enable me to attach great importance to the 

 resulting figures. Nevertheless, I obtained values which strengthened the 

 conclusions to which I was led by the' more exhaustive examination of the 

 dilute solutions of sugar and potassiuol chloride. 



So far back as the Liverpool Meeting of this Association I expressed a hope 

 that the experimental difficulties of the direct measurement of osmotic pressures 



