338 Messrs. Wright and Rennie on the Determination of 



latent heat of water-vapour at 15° being nearly 600, this 

 would represent very nearly 8 gramme-degrees per litre of 

 evolved gases. Inasmuch as the gases were evolved not from 

 pure water but from water containing 22 per cent, of sul- 

 phuric acid, it is likely that the gases actually evolved con- 

 tained slightly less moisture than the normal saturating 

 amount ; but as in most of the experiments only about 500 

 to 550 cub. centims. of gases were evolved, the total correc- 

 tion is only about 4 gramme-degrees, and a trifling error in 

 its estimation is negligible. 



In the next place a minute amount of heat is absorbed in 

 separating from sulphuric acid the water electrolyzed. From 

 the experiments of Favre and Quaillard (Comptes Bendus, 1. 

 p. 1150), 98 grammes of sulphuric acid already mixed with 216 

 of water (forming a dilute acid of composition H 2 S0 4 + 12H 2 0) 

 evolve 483 gramme-degrees when 72 grammes more of water 

 are added (forming H 2 S0 4 + 16H 2 0). The further addition of 

 72 grammes more of water evolves 222 gramme-degrees, whilst 

 the addition of a third 72 grammes (forming H 2 S0 4 + 24H 2 0) 

 evolves 141 gramme-degrees. From these numbers, by in- 

 terpolation, it results that acid of strength H 2 SO 4 + 20H 2 O 

 (containing 22 per cent, of H 2 S0 4 ) will evolve close upon 

 2*5 gramme-degrees when 1 gramme of water is added to such 

 a quantity of acid as not sensibly to alter its composition by 

 dilution ; i. e. that for every gramme of water decomposed by 

 electrolysis 2*5 gramme-degrees are absorbed. 



In the experiment the calorimetric details of which have 

 just been given, the corrected heat-evolution finally becomes 

 as follows :■■ — 



gramme-degrees. 

 Heat-evolution calculated from rise of ther- \ 0990 



mometer corrected for radiation &c. . . J 

 Correction for heat absorbed by moisture in^ 



0*4967 gramme of mixed oxygen and hy- > 9*2 

 drogen evolved during experiment . . .) 

 Correction for heat absorbed in separating^ 



0*4967 gramme of water from dilute sul- > 1*2 

 phuric acid J 



3338-4 



45. The value of E, the average difference of potential be- 

 tween the voltameter plates, was found thus. Before the experi- 

 ment commenced the scale of the electrometer was standardized 

 by means of a Clarke cell, several readings being taken by re- 

 versal so as to obtain total readings each equal to double the 



