during the Combination of Acids and Bases. 235 



combined with an equivalent of different acids, nearly the same quantity of 

 heat ; but different bases a different quantity. 



Law 2. — When a neutral is converted into an acid salt., by combining 

 with one or more atoms of acid, no change of temperature occurs. 



Law 3. — When a neutral is converted into a basic salt, by combining with 

 an additional proportion of base, the combination is accompanied with the 

 evolution of heat. 



24. To the first of these laws important exceptions are presented by the 

 peroxide of mercury among the bases, and by the hydrocyanic, and probably the 

 carbonic acid, among the acids ; and it is not improbable that more extended 

 investigations will lead to the discovery of other exceptions. The second law 

 has been established by numerous experiments, and can scarcely be said to be 

 liable to any well-marked exception ; but I feel much less confidence in enun- 

 ciating the third, as a general principle, from the very limited number of cases of 

 soluble subsalts in which it was possible to verify its accuracy. 



25. In order to obtain results of as much uniformity as possible, the standard 

 alcaline solution was always mixed with rather a greater quantity of acid than 

 was necessary to neutralize it.* The combination was thus effected more rapidly 

 and certainly, than if an attempt had been made to form an exactly neutral 

 compound. That this excess of acid did not produce any sensible difference in 

 the result, will be rendered evident, when the experiments are examined, which 

 will be hereafter cited, in illustration of law second ; and, indeed, if no basic 

 compound existed, the numbers obtained were identical, whether an equivalent 

 of base was neutralized by an excess of acid, or a like equivalent of acid neutralized 

 by an excess of base. I have arranged, in distinct tables, the increments of 

 temperature obtained by combining an equivalent of each base with the acids. 

 The equivalents taken were .353 grammes potash, .234 gr. soda, .129 gr. 

 ammonia, .572 gr. barytes, .213 gr. lime, .154 gr. magnesia, .301 gr. oxide of 

 zinc, .834 gr. oxide of lead, .870 gr. oxide of silver, and .810 gr. peroxide of 



* In the cases of the phosphoric and arsenic acids, the quantity of acid was just sufficient to con- 

 vert the alcali into the common phosphate and arseniate ; that is, half an equivalent of acid for an 

 equivalent of base. The reason of this will appear again (35). The number for chromic acid is 

 only deduced from an indirect experiment upon the bichromate of potash. 



2h2 



