518 Dr. Andrews's Report on the Heat of Comhination. 



scribes an extensive set of experiments on the heat evolved during 

 the union of certain bases with acids of different degrees of concen- 

 tration. These experiments serve to illustrate the general principle, 

 that in the formation of a chemical compound the heat developed 

 is a constant quantity, being the same in amount, whether the 

 combination takes place directly at one time or indirectly at re- 

 peated times. Thus he finds that on neutralizing an aqueous solu- 

 tion of ammonia with sulphuric acid, containing one, two, three and 

 six atoms of water, there is a different development of heat in each 

 case ; but by adding to the results found by experiment in the three 

 latter cases the quantities of heat due to the combination of the 

 nionohydrated acid, with one, two and five atoms of water respect- 

 ively, the same number is obtained in each case as in the direct 

 combination of the monohydrated acid itself. This principle is 

 correct, but it is almost self-evident and scarcely required so ela- 

 borate a proof. 



The bases examined by Hess were potash, soda, ammonia and lime, 

 which he combined in different ways with the sulphuric, nitric and 

 hydrochloric acids. The conclusion at which he arrives is, that the 

 same acid in combining with equivalents of different bases produces 

 the same quantity of heat, but at the same time he expresses some 

 doubt as to the applicability of this principle to all similar cases of 

 combination. Indeed his own experiments with lime and ammonia 

 do not accurately agree with it ; I refer particularly to his experi- 

 ments with ammonia, which, when properly interpreted, appear to 

 me to prove clearly that that base in combining M'ith acids developes 

 less heat than potash or soda, although I am aware that Hess himself 

 has drawn from them a different conclusion. 



About the time of the publication of the first part of Hess's me- 

 moir, I had completed an investigation of the same subject, but instead 

 of employing strong solutions of the acids and bases, I diluted all the 

 liquids largely with water previous to examining their thermal reac- 

 tions. In this way I hoped to avoid the complex effects that arise 

 when successive combinations and decompositions of different kinds 

 occur in the same chemical action, and the result fully realized my 

 anticipations. The general conclusion deduced from this investi- 

 gation may be briefly expressed, by stating that the heat developed 

 during the union of acids and bases is determined by the base and 

 not by the add. The following special laws will be found to com- 

 prehend the greater number of cases of chemical action to which 

 the foregoing principle can be made to apply. 



1. An equivalent of the same base, combined with different acids, 

 produces nearly the same quantity of heat. 



2. An equivalent of the same acid, combined with different bases, 

 produces different quantities of heat. 



3. When a neutral salt is converted into an acid salt by cortibi- 

 ning with one or more equivalents of acid, no disengagement of heat 

 occurs. 



4. When a double salt is formed by the union of two neutral salts, 

 no disengagement of heat occurs. 



