Heat evolved in Chemical Combination. 181 



be asked how lime differs so much from the other bases, which 

 give about 600. This may depend upon two things; Jirst, that 

 the two atoms of water, which gypsum contains, evolve heat, 

 or else it could not be a chemical combination ; or, secondly, 

 that it may arise from the solidification of the sulphuric acid. 

 In order to obtain some indications on this point, M. Hess 

 made two experiments on the quantity of heat evolved by 

 gypsum in setting, and found that for S O a there are evolved 

 36-5 to 37*7 units of heat. 



If we subtract that from the 64-2*5, we have 604*8 for the 

 heat evolved with oil of vitriol, or 1076*1 with anhydrous sul- 

 phuric acid. 



M. Hess also ascertained that dry lime, in combining with 

 water, evolves 167*2 units of heat. 



Experiments with Muriatic Acid. 



M. Hess employed the acid H CI + 12 HO, which con- 

 tains 0*252 of dry acid, and has sp. gr. 1*125. This acid still 

 evolves heat when mixed with water, and the quantity being 

 determined, as in the case of sulphuric acid, was found to be 

 50*84 units of heat for one atom of muriatic acid. In calcu- 

 lating this result, however, the atom of sulphuric acid is still 

 taken as unit, which makes the atom of muriatic acid 0*908. 



The acid H CI + 12 H O (previously diluted) being satu- 

 rated with ammonia, gave out 318*8 units of heat; and in an- 

 other trial exactly the same number was obtained. 



Three experiments were made by means of the acid HCI + 

 12 H O, the water and the ammonia being all mixed at the 

 same time; the heat evolved was 367*7, 364*9, and 374«*7, 

 giving a mean of 369*1. If we then take the former result, 

 we have 318*8 + 50*S4 = 369*64. 



The acid H CI + 12 HO, mixed with water and potash, 

 evolved 361*9 units of heat. 



The acid H CI + 12 HO, mixed with water and soda, 

 evolved 376*4 units in one and 360* units in another expe- 

 riment. 



It has been already remarked, that the general principle of 

 the quantity of heat evolved in combustion being proportional 

 to the quantity of oxygen absorbed, was supported, at least 

 approximately, by the example of carbon, in forming carbonic 

 oxide and carbonic acid. This subject has, however, given 

 rise to some difference of interpretation of Dulong's number, 

 given in page 20. M. Hess, who considers that carbonic 

 oxide contains its own volume of gaseous carbon, deduced 

 therefrom, that the quantity of heat evolved in forming car- 



