Dr. Andrews's Report on the Heat of Comhination. 519 



5. When a neutral salt is converted into a basic salt, the com- 

 bination is accompanied by the disengagement of heat. 



6. When one and the same base displaces another from any of its 

 neutral combinations, the heat evolved or absorbed is always the 

 same whatever the acid element may be. 



As some of the bases (potash, soda, barytes and stroutia) form 

 what we may perhaps designate an isothermal group, such bases 

 will develope the same, or nearly the same heat in combining with 

 an acid, and no heat will be developed during their mutual displace- 

 ments. 



These laws are not intended to embrace the thermal changes 

 which occur during the conversion of an anhydrous acid and base 

 into a crystalline compound. The steps by ^hich such a conversion 

 is effected are generally very complicated, and involve successive 

 combinations and decompositions. We cannot combine, at ordinary 

 temperatures, a dry acid and a dry base; and when combination 

 takes place in presence of water, iiydrates of the acid and base are 

 first formed, which are afterwards decomposed, and the crystalline 

 salt finally obtained is sometimes anhydrous, sometimes combined 

 with water. To expect simple results where so many different ac- 

 tions must produce each its proper thermal effect, would be alto- 

 gether vain, and to introduce the consideration of some of these 

 actions without the whole would only render the numbers empirical. 

 In the experiments from which the foregoing laws were deduced, the 

 acids and bases before combination, and the compounds after com- 

 bination, were as nearly as possible in the same physical state. The 

 only change which occurred was the combination of the acid and 

 base, and the heat evolved must therefore have arisen from the act 

 of combination. Such changes of temperature as are produced by 

 solution are not in any way concerned in producing these thermal 

 effects, as none of the reacting bodies assumed at any time the solid 

 state. The insoluble bases form, it is true, an unavoidable exception 

 to this statement, and in the experiments with them, the results 

 would require to be corrected for the heat due to the change of the 

 base from the solid to the fluid state. As this correction, however, 

 although unknown, must be a constant quantity for the same base, 

 it would not, if applied, interfere with the direct proof of the first law. 



In an inquiry of this kind, it is important, while endeavouring to 

 generalize the results of experiment, to point out at the same time 

 the differences which occur in particular cases between those results 

 and the numbers deduced from the theory. In the whole range of 

 tlie science of heat, scarcely a single general principle has yet been 

 discovered which is strictly in accordance with all the results of 

 experiment ; and from the application of improved methods of expe- 

 rimenting, discrepancies of this kind have of late years been found 

 to exist where they had not before been suspected. 



In the original experiments from which the first of the foregoing 

 laws was deduced, the mean heat developed by the nitric, phosphoric, 

 arsenic, hydrochloric, hydriodic, boracic, chromic and oxalic acids 

 being 6°*61, the greatest deviation from the mean on either side 



