412 Mr. Graham on the Heat disengaged in Combinations. 



b"b" 



change of temperature was perceived on mixing dilute solu- 

 tions of a magnesian and potash sulphate; and one of these 

 salts was dissolved in a solution of the other with the same fall 

 of temperature as in pure water. Although I think it all but 

 certain that these salts combine at once on mixing, I could 

 not discover a single circumstance which was decisive of the 

 fact. The density of such a mixture of salts was not altered 

 by boiling it alone or with spongy platinum, and was exactly 

 the same as that of the liquid formed on dissolving in water a 

 corresponding quantity of the crystallized double sulphate. 

 The addition of an equivalent of sulphuric acid already highly 

 diluted to each of the solutions thus compared, produced 

 exactly the same fall of temperature. On the other hand, 

 this fall of temperature was as nearly as possible the same 

 as that obtained on dividing the acid into two equal portions, 

 and mixing separately a solution of each of the constituent 

 salts with each portion. The solution of a double salt appears 

 therefore to be as nearly as possible equivalent to the consti- 

 tuent salts dissolved apart. Even in the formation of alum 

 no certain change of temperature was observable ; one-fourth 

 of an equivalent of sulphate of potash (13*63 grains), when dis- 

 solved in 1000 grains of water, producing a fall of 0, 32 R., 

 while when dissolved in 1000 grains of water containing one- 

 fourth of Al 3 3 + 3 S0 3 , the fall was 0°'35, 0°32, 0°35 R., 

 of which the mean is 0, 34 ; the experiments being made at 

 57° F. 



But these double sulphates being all less soluble than their 

 constituent sulphates, it was desirable to make the experiment 

 upon the formation of a double salt, which is more solu- 

 ble than its constituents ; such as the double chloride of mer- 

 cury and ammonium. One-half of an equivalent of chloride 

 of mercury, 42'70 grains, was dissolved in 1544 grains of 

 water at 64° F., with a fall of 0°-29, 0°-30, 0°"30 F., of which 

 the mean is o, 30 F. The same quantity of chloride of mer- 

 cury was dissolved in 1544 grains of water, containing half 

 an equivalent, 16*74 grains, of chloride of ammonium, at 

 63° F., with a fall of 0°'13, 0°-12, 0°*12 F., of which the 

 mean is 0°*12 F. Doubling these results, we have the fall 

 from a whole equivalent of chloride of mercury in water equal 

 to o, 60 ; from chloride of mercury in chloride of ammonium 

 o, 24 ; the difference, or 0°*36, being due to heat evolved in 

 the formation of the double salt. The latter, however, or 

 sal-alembroth, assumes an atom of water of crystallization in 

 its formation, which may perhaps occasion some change of 

 temperature. 



When 1 equivalent of chloride of mercury was dissolved 



