512 Dr. Andrews's Report on the Heat of Combijiation. 



effects that occur in chemical actions of every kind. A iew new 

 experiments will Jje described in their proper places. These will be 

 given in some detail, but when referring to experiments already 

 published, all numerical quantities will, as far as possible, be avoided. 

 Before entering upon the consideration of chemical combinations 

 and decompositions properly so called, it may be useful briefly to 

 refer to the thermal changes which accompany solution. The earlier 

 experiments on this subject having been made solely with the object 

 of discovering frigorific mixtures, do not furnish quantitative mea- 

 sures of any scientific value. But of late years the inquiry has been 

 pursued in a more useful way by GayLussac, Thomson, Karsten, 

 Chodnew and Graham. The salts examined have been chiefly the 

 soluble sulphates, niti'ates and chlorides, and the solvents pure water 

 and saline and acid solutions. The principal results of these inves^o 

 ligations I have endeavoured to express in the following proposi^'- 

 tions : — 



1. The solution of a crystallized salt in water is always accom--. 

 panied by an absorption of heat. >'J 



2. If equal weights of the same salt be dissolved in succession in 

 the same liquid, the heat absorbed will be less on each new addition 

 of salt. 



3. The heat absorbed by the solution of a salt in water holdings 

 other salts dissolved, is generally less than that absorbed by its solu^b 

 tion in pure water. x;s 



4. The heat absorbed by the solution of a salt in the dilute mineral 

 acids, is generally greater than that absorbed by its solution in water. 



As the subject is of great extent and the inquiry has hitherto, 

 embraced only a small number of cases of solution, it is not unlikely 

 that some of these conclusions will require hereafter to be modified. 

 From some experiments by Graham on the solution of salts belonging 

 to certain isomorphous groups, there is reason to suspect the exist- 

 ence of a connexion between isomorphism and the absorption of heat 

 in solution. hr 



The foregoing remarks apply only to the solution of crj'stallized 

 Baits. If, however, we take a salt which crystallizes with water and 

 make it anhydrous before solution, the thermal results will be alto- 

 gether different. The anhydrous salt, when added to an excess of 

 water, will first combine with its ordinary equivalent of water of cry- 

 stallization, and the new compound will then dissolve. The change 

 of temperature observed is therefore a complex quantity arising from ■ 

 the heat of combination due to the union of the anhydrous salt with cl 

 water, and the heat absorbed by the solution of the hydrous salt.t'^ 

 From a comparison of the results obtained on dissolving the same 

 salt in the anhydrous and hydrous states, Graham has endeavoured tout 

 deduce the amount of heat due to the combination of the dry salt with 

 its water of crystallization. According to his experiments, the sul- 

 phates of water, copper and manganese, disengage the same quantity^^lit 

 of heat in combining with the first atom of water. The sulphates^jj^ 

 of magnesia and zinc also disengage equal quantities of heat in their.or, 

 complete hydration. The same simple relation is not however ob* 



