446 



Mr. E. H. Griffiths on 



Dietekici. — These experiments are characteristic examples 

 of the care and accuracy by which Dieterici's work is distin- 

 guished, and possess one great feature of excellence, i. e. their 

 results are much less dependent than in previous cases on the 

 thermometers used by the observer, since no change of tem- 

 perature takes place during an experiment. One correction, 

 however, is necessary in his value of J (4*2436 x 10 7 ). The 

 resistance of his conductor w r as measured in terms of a legal 

 ohm, and the correction to the Board of Trade or " true ohm" 

 reduces this value to 4*233 x 10 7 , expressed in terms of the 

 mean calorie. It must be remembered, however, that Bunsen, 

 Schuller and Wartha, and Yelten differ by 1 part in 250 

 regarding the value of a constant on which the numerical 

 value obtained by Dieterici is dependent. 



My own Experiments. — Naturally I can here say but 

 little regarding my own work. I would, however, venture to 

 point out in what I consider the strength of that determination 

 lies. Two entirely different systems of thermometry were 

 used, viz. : a platinum-thermometer scale (the reduction to 

 the air- thermometer being based, on the previous observations 

 by Professor Callendar and myself) ; also the thermometers 

 thus standardized were compared, under the conditions in which 

 they were used, with the nitrogen-scale of the Bureau Inter- 

 national, when the range-value was found to be practically 

 identical and the actual elevation, as determined by the two 

 methods, differed only by 0°*005. When it is remembered how 

 extremely dissimilar were the two methods of standardization, 

 the close agreement between the results greatly strengthens 

 the conclusions. Again, the nature of the experiments enabled 

 me to vary the conditions to a greater extent than w r as found 

 possible by other observers, and thus any constant errors would 

 probably have been detected. That this was the case was 

 shown by the persistent failures (during the years 1888 to 

 1891) to obtain consistent results when the conditions were 

 thus altered. The weakness of those experiments w r as that 

 small masses of water were used. These masses were, how- 

 ever, chosen deliberately, as any increase would have led to 

 the accumulation of other causes of error *. The value 

 resulting from these experiments was given as 4*198 in terms 

 of a thermal unit at 15° C.f Professor Schuster has since 

 pointed out a necessary correction of 1 in 4000 due to the 

 specific heat of the air displaced by the water. This would 

 give 4*199 X 10 7 . In my original paper Mr. Clark and I stated 

 our impression that in assuming the mean E.M.F. of our cells 

 as 1*4344, instead of 1*4342, we were possibly in error, as there 



* Trans. Roy. See. vol. clxxxiv. A. p. 415. 

 t Proc. Roy. Soc. ; January 18D4. 



