Latent Heat of Evaporation of Benzene. 25 



and 5 cells, and we regard the agreement amongst the results 

 as very satisfactory, and as establishing the validity of the 

 various corrections. In thermal investigations it is as a rule 

 difficult to alter all the conditions in so complete a manner as 

 that caused by the changes above referred to. For example, 

 if any " priming" had taken place (the usual cause of inaccu- 

 racy in determinations of the Latent Heat of Evaporation), 

 its effect must have been greatly increased when the rate of 

 evaporation was nearly trebled, which was the case when the 

 potential-difference was altered from 3 to 5 cells. Again, 

 the importance of the different terms undergoes such changes 

 that any constant error in the determinations of Q s ^ s and %q 

 would cause the values of L when n = 4= to lie between the 

 values when n = 3 and w = 5. An inspection of the Tables 

 will show that no such effect is visible. 



We also varied the " stirring supply " very considerably. 

 The Tables show that (at the same temperature) the values of 

 Q s have been changed from '00276 (Expt. XVII.) to -00427 

 (Expt. XXIV.). 



\_Note hy E. H. Griffiths. — My experience with apparatus 

 of a similar nature to that used in these experiments has con- 

 vinced me of the severity of the test above referred to. 

 During my attempts at a determination of the mechanical 

 equivalent during the years 1887-1891 the results invariably 

 broke down when thus tested. The following quotation, refer- 

 ring to the experiments anterior to 1892, is from p. 364, 

 Paper J : — 



"The agreement amongst individual experiments taken 

 under the same conditions, was, if anything, at times more 

 marked than in the experiments of 1892 ; nevertheless, when 

 the final reduction took place, fatal discrepancies invariably 

 showed themselves. . . . Experiments conducted with a high 

 electromotive force invariably gave too great a value for the 

 time as compared with that obtained when a lower electro- 

 motive force was used." 



It appears to me that it is this power of altering all the 

 conditions which renders electrical methods of such great 

 value when applied to thermal determinations.] 



