Latent Heat of Evaporation of Benzene. 5 



differences d' and d" between the temperature of the calori- 

 meter and the surrounding walls, we may conclude that the 

 whole of the heat thus evolved by the calorimeter was ex- 

 pended in the evaporation of water, hence 



%=<W-*o") +(<*'-<*")}*• 



(3) 



Hence 



2 2 j. 



ML=^Ji+Q s x t s + G ti {W-0 o ")+(d'-d")}. (4)" 



3. The following table shows the comparative mean values 

 of the terms in equation (4) resulting from our experiments 

 on benzene f . 



Table I. 



No. of Cells. 



Qe*e- 



Qs^s- 



22. 



5 



1 



■008 



±•005 



4 



1 



■on 



±•009 



3 



1 



•015 



±•001 



We found that the thermal balance was most easily main- 

 tained when the potential-difference was that of 4 cells, there- 

 fore the majority of our experiments were performed with 

 that potential-difference, the experiments with 5 and 3 cells 

 being used as a check upon the results. To secure an accu- 

 racy of (say) 1 in 1000 in the total heat-supply, the above 

 table shows that it was necessary to measure 



Q E £ B to (say) 1 part in 2000, 

 Qs* s » 1 » 15 > 

 2? „ 1 » 10; 



but as the sign of the last term was in some experiments 

 positive, in others negative, the above degree of accuracy was 



* This apparently clumsy method of representing the quantity of heat 

 evolved or taken up by the calorimeter was adopted because, as the 

 method of experiment involved separate determinations of 8 Q ' } O ", d', and 

 d", the actual temperature of the calorimeter at any time could only be 

 obtained in this manner. 



t (Experiments Preliminary, XIX. a, XXXV., and XXXVI. were not 

 included when calculating this table, as they are in several respects 

 exceptional.) 



