Mr. Graham on the Constitution of Sulphates. 539 



In a platinum crucible of larger size the effects wex'e not so 

 marked, 34'° Fahr. being the greatest difference obtained; 

 this of course would arise from the greater bulk of the melted 

 metal not exposing comparatively so large a surface to the 

 coolinff medium. 



LXXIX. On the Constitution of the Sulphates as illustrated 

 by late Thermometrical Researches. By Thomas Graham, 

 Esq., F.R.S., 4-c.* 



T3ROF. Hess and Dr. Andrews both apply the results of 

 -*- their late inquiries respecting the heat evolved in combina- 

 tion f to test the accuracy of a view of the constitution of dou- 

 ble and acid salts which was published by myself, and arrive, 

 it is remarkable, at opposite conclusions. 



The view in question, I may first state, taking the example 

 of double and acid sulphates. Crystallized sulphate of magne- 

 sia, and the double sulphate of magnesia and potash, I have 

 represented thus : — 



Mg O, S O3 (H O) + 6 H O, 

 Mg O, S O3 (K O, S O3) + 6 H O; 

 considering the latter salt to be derived from the former, by 

 the substitution of sulphate of potash for that single atom of 

 water, which is found to be much more strongly attached to 

 the sulphate of magnesia than the other six. This atom of 

 water, which is not basic water, was formerly named saline 

 water, to indicate that it is replaceable by a salt; its presence 

 being considered a provision in sulphate of magnesia for 

 the formation of double salts. The water and sulphate of 

 potash are therefore looked upon as equivalent in the con- 

 struction of the two salts; and the substitution of the salt for 

 the water might therefore be reasonably expected to occur 

 without the evolution of heat. 



In accordance with that statement. Dr. Andrews finds that 

 no heat is evolved on mixing solutions of sulphates of magnesia 

 and potash, nor in the formation of any other double salt. 

 On repeating the experiment I found also no heat nor change 

 of temperature on mixing the solutions, although a change of 

 „Lth of a degree Fahr. would have been distinctly indicated 

 by my thermometer. 



Possibly, however, the double salt may not immediately be 

 formed, and hence no change of temperature at the moment 

 of mixing the two solutions, nor for some time afterwards. To 



* Communicated by the Clicmical Socict}' ; having been read January 18. 

 f Fhil. Mag., January 1842. 



2 02 



