the Thermal Unit. 449 



In addition to the above we have Dieterici's value after 

 correction to the "true ohm," viz. 4*233 in terms of the "mean 

 calorie." No purpose would be gained by endeavouring 

 to express Dieterici's results in terms of the same unit as the 

 others, for, as I have endeavoured to show, the evidence 

 regarding the relation between the " mean calorie " and the 

 " thermal unit at 15° " is unfortunately so contradictory that 

 no comparison is possible until further light is thrown on the 

 subject. 



Plate VII. shows, more clearly than the table, the relation 

 between the different determinations. The dotted lines 

 indicate the values deduced by the above assumptions with 

 regard to the changes in the specific heat of water. 



I quite agree with Professor Schuster's conclusion"* that 

 greater value should be attached to results based on mechan- 

 ical rather than on electrical methods. He thinks it pro- 

 bable that the correct value lies somewhere between Joule 

 and Rowland. When we remember, however, the very small 

 rises of temperature on which the conclusions of Joule are 

 based, and the inevitable uncertainty connected with the re- 

 standardization of his thermometers after so great a lapse of 

 time, we should, I think, give far the greater weight to 

 Rowland's conclusions. 



Many arguments can be adduced in support of the elec- 

 trical method of investigation. The experimental difficulties 

 are less than when other methods are adopted, but on the other 

 hand the results are dependent on the validity of our values of 

 the electrical units. I felt this so strongly that I endeavoured 

 so to phrase the title of my own paper on this subject as to 

 show that I regarded that work as an effort to " establish the 

 relation between the electrical and mechanical units," and to 

 trace the " changes in the capacity for heat of water," rather 

 than as a direct determination of J. I trust that I shall not be 

 considered too presumptuous if I state that some doubts exist 

 in my own mind as to the absolute accuracy of the electro- 

 chemical equivalent of silver. A change of 1 in 1000 in that 

 constant would bring into very close approximation the re- 

 sults obtained by Schuster and Gannon, Rowland, and my- 

 self ; for, as a consequence, the number representing the 

 electromotive force of a Clark's cell would be also changed. 

 No such alteration, however, could make the agreement abso- 

 lute owing to the different rate of change with temperature 

 indicated- by the experiments by Rowland and myself ; our 

 differences, therefore, must be partially due to thermometry, 

 for no alteration in the constants involved in the work of 



* Phil. Mag., June 1805, p. 501. 



