August 9, 1906 J 



NA TURE 



359 



ithe residue would possibly tlirow light on many problems 

 now awaiting solution. 



It would appear that if we made a similar imprison- 

 ment of uranium, a like degree of accuracy would not be 

 attainable until after the lapse of half-a-million years, and 

 I am afraid that our interest in the work of our successors 

 cannot be expected to cover so long a period. Neverthe- 

 less, it is probable that the presence of the products of 

 decomposition could easily be detected after the lapse of a 

 comparatively short interval of time. 



The experiment might well be extended so as to include 

 examples of all the elements capable of such treatment ; 

 and with each prisoner should be placed a full record of 

 its physical constants, such as mass, density, electrical 

 conductivity, specific heat, S:c., with a clear indication of 

 what is believed to be the probable accuracy of such de- 

 termination. 



During the past twenty-live years much thought has been 

 devoted to the accurate determination of certain physical 

 constants. This is very apparent in the case of one of 

 the most important — namely, that commonly termed the 

 " mechanical equivalent of heat," or, as I prefer to define 

 it, the "thermal equivalent of energy." When Lord 

 Kelvin addressed you in 1881, I think it probable that he 

 would have indicated the value obtained by Joule — viz., 

 7726 foot-pounds — at Manchester, as the quantity of work 

 required to raise the temperature of one pound of water 

 through 1° F. at 62° F. It is true that the results of 

 Rowland's classical investigation were published in 1880 

 and 1881, but the discrepancy between his conclusions and 

 those of Regnault regarding the change in the specific heat 

 of water at temperatures between 0° C. and 30° C. intro- 

 duced an element of uncertainty. 



As a consequence of this discrepancy much experimental 

 work on the subject has been performed in the last 

 quarter of a century, and I think it may be said without 

 hesitation that the value of this important constant is now 

 ascertained with an accuracy of about one part in 2000. 

 The amount of labour which has been employed in the 

 determination of this thermal constant is extraordinary, 

 and, as I have pointed out elsewhere, it well illustrates 

 the cosmopolitan character of scientific investigation. 



I have given reasons (Griffiths, " The Thermal Measure- 

 ment of Energy ") for specially selecting for consider- 

 ation the determinations of Rowland, of Bartoli and 

 Sfracciati, of Ludin, of Callendar and Barnes, of Schuster 

 and Gannon, and I have ventured to add my own. Thus 

 Baltimore, Pisa, Zurich, Montreal, Manchester, and Cam- 

 bridge have all contributed to the solution of the problem, 

 and we may now with some certainty say that 7777 foot- 

 pounds at Greenwich are very closely the equivalent of 

 the amount of heat required to raise i lb. of water through 

 1° on the hydrogen scale at 63°-5 F. 



It may possibly appear that the result just quoted is a 

 somewhat poor return for the expenditure of so much 

 thought and labour. I would call attention, therefore, to 

 the fact that the value of this equivalent is dependent on 

 the measurements of many other natural constants ; hence 

 any agreement between the results obtained by the observa- 

 tions of Rowland and some of the other observers I have 

 mentioned would only be possible in the absence of errors 

 of appreciable magnitude in the determinations of mass, 

 of change of temperature, and of electrical resistance and 

 current. Certain discrepancies have led to the discovery 

 of a hitherto unsuspected cause of inaccuracy, especially in 

 the determination of temperature, and thus the inquiry 

 has rendered valuable service in many branches of physical 

 inquiry. 



For example, so far back as i8q3 I ventured upon a 

 prophecy that the value assigned to the E.M.F. of a Clarke's 

 cell was somewhat too high, and that it was possible that 

 1-4328 represents more truly the potential difference of a 

 Clarke's cell at 15° C. than the ordinarily accepted value of 

 1-4342. In the report of the Electrical Standards Com- 

 mittee for 1897 w'll t'B found a discussion of this matter, 

 and one of the consequences of the deliberations of that 

 Committee is to be seen in the ampere balance now stand- 

 ing in the National Physical Laboratory. 



The results of the observations obtained by this instru- 

 ment will, I believe, shortly be published by Prof. 



NO. I9I9, VOL. 74] 



.\yrton and Mr. Mather, but I am at liberty to state that, 

 so far as the observations have been reduced, they point 

 to the conclusion that 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 

 b.isis of our system of electrical measurement are not only 

 practicallv determined with a high degree of accuracy, but 

 that also our measurements of tcTiiperature 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 applications of these discoveries 

 to the purposes of mankind have been evident and imme- 

 diate. 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 discovery of Hertzian waves. 



The ambition of the student is naturally fired by such 

 e>:amples, and there is a possible danger that the plodding 

 but absolutely necessary work of accurate measurement 

 mav suffer by neglect. I therefore venture to repeat the 

 well-established axiom that our advance in scientific know- 

 ledge is a function of accurate measurement, and that the 

 student who devotes his energy to the determination of 

 some physical constant is probably giving a " point of de- 

 parture " to the pioneer. For it must ever be remembered 

 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 1-251, 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, 

 l-'or all we know the 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 the exist- 

 ence of which appears necessary for its violation. 



Mr. Campbell recently expressed doubts as to the ap- 

 plication of thermodynamic considerations to osmetics. 

 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 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 evi- 

 dence of this much-debated matter could be obtained from 

 the observation of the freezing-point of a very dilute solu- 

 tion that I embarked on a series of somewhat elaborate 

 experiments 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 0003 to 0025 gm. -molecule per litre. 



Subsequently to my departure from Cambridge Mr. 

 Bedford re-erected the apparatus in another building. After 

 having surmounted great difficulties, he repeated many of 

 mv experiments, and he informs me that the numbers he 

 has so far obtained are in almost entire agreement with 

 those previously obtained by me. The molecular depres- 

 sion in the case of cane sugar I found to be rSsS, of 

 potassium chloride 3 720, and I understand that Mr. 

 Bedford's experiments agree with these results with a dis- 

 crepancy of less than i part in 1000. The most probable 

 number obtained from theoretical considerations would be 

 in the former case 1-S57, in the latter 3714. As Mr- 

 Whetham has pointed out, unless there is some balancing 



