C hemic al Affinity in terms of Electromotive Force. 257 



If, however, the value of J is included in the value of €, as is 

 necessarily the case when e is calculated by the formula 

 £=E — H^J (§ 11), this formula becomes 



whence 



25. In the case of water, a number of determinations of h 

 (the heat evolved by the union of oxygen and hydrogen by 

 combustion) have been made by various observers, but for the 

 most part with slightly different values for the heat unit. 

 Leaving out of sight the first approximate determinations of 

 Dulong, Hess, and Grassi (respectively 34,743, 34,792, and 

 34,666 gramme-degrees per 9 grammes of water formed), the 

 following comparisons of these different determinations may 

 be made, reducing them all, as far as possible, to the same 

 conditions — viz. supposing the oxygen and hydrogen origi- 

 nally used per gramme-equivalent of water (respectively 7*98 

 and 1 gramme) and the water produced (8*98 grammes) to be 

 all at 0° and 760 millims., the value of a gramme-degree being 

 defined as the heat required to raise 1 gramme of water from 

 0°tol°C. 



The most recent of these determinations is that of Schuller 

 and Wartha. By introducing hydrogen and oxygen at 0° into 

 a slightly modified Bunsen's ice-calorimeter, burning them, 

 and weighing accurately the water formed, these observers 

 found (Pogg. Ann. [2] ii. p. 359), as the mean of four closely 

 concordant valuations, that when 8*98 grammes of water are 

 produced from 1 gramme of hydrogen and 7*98 of oxygen, the 

 materials and products formed being all at 0° and 760 mil- 

 lims., the heat produced is sufficient to raise 341*261 grammes 

 of water from 0° to 100° ; i. e. defining what they term a 

 " mean calory " to be y^ of the heat required to raise a 

 gramme of water from 0° to 100°, the heat developed is 

 34126*1 "mean calories." According to Eegnault (Memoires 

 Acad. Sciences, 1847, vol. xxi.), the specific heat of water at 

 various temperatures is given by the formulae 



Q = t + 0*00002 t 2 + 0*0000003 1\ 

 S = 1 + 0*00004* + 0*0000009 1 2 ; 



where Q is the quantity of heat required to raise a unit of 

 weight of water from to t, and S is the specific heat at t°; 

 so that the mean specific heat between 0° and 100° (i. e. the 



