540 Mr. Graham on the Constitution of Sidjphates. 



meet this objection, solutions of sulphate of magnesia and of 

 sulphate of ammonia (the last, from its greater solubility, being 

 preferred to sulphate of potash) were made of such a strength 

 that they might be mixed without the precipitation of the 

 double salt immediately occurring, but strong enough to allow 

 a large quantity of the double salt to fall upon stirring the li- 

 quid strongly. The solutions were ISiG'SS grains of cr. sul- 

 phate of magnesia dissolved in so much water as to form 8000 

 water grain measures, and 613*5 grains oil of vitriol, neu- 

 tralized with ammonia and made up to 4000 water grain mea- 

 sures. On mixing one ounce measure of the first with half an 

 ounce measure of the second, both exactly at 50°, not the small- 

 est change of temperature could be observed ; but as soon as 

 the double salt began to deposit, the temperature rose, and on 

 stirring strongly much salt was deposited, and the temperature 

 rose 5'40° Fahr. On re-dissolvrng this salt, however, by 

 substituting for the mother liquor an equal bulk of water, the 

 temperature instantly fell 5'85°. Hence the heat which first 

 appeared was produced by the solidification of the double 

 salt, and disappears upon its liquefaction. There is no heat 

 referable to combination of the two salts. The cold on dis- 

 solving was always somewhat greater than the heat on pre- 

 cipitating the double salt, in repetitions of this experiment, 

 chiefly, I believe, from the slowness of the precipitation, which 

 requires a minute or two, so that a portion of the heat is lost 

 from contact with the atmosphere, and the whole not observed, 

 while the subsequent solution of the salt being almost instan- 

 taneous, the whole fall of temperature is observed. The same 

 experiment was made with a solution of sulphate of zinc, of 

 the same strength as the sulphate of magnesia, and with simi- 

 lar results, only that the fall of temperature, on solution, was 

 somewhat less than that on solidification, namely, as 9°'22 to 

 9°-67, difference 0°*45 Fahr. This was principally owing to 

 the time required in re-dissolving this double salt being greater 

 than that occupied in precipitating it, three applications of 

 water being required to re-dissolve the double salt completely, 

 owing to its sparing solubility. 



M. Hess's objection is made to the analogous constitution 

 which I have assigned to the bisulphate of potash : — 

 Sulphuric acid of specific gravity 1*78, H O, S O.3 (H O). 



Bisulphate of potash . . . H O, S03(K0,S0J. 



He maintains that heat is evolved in the formation of a bi- 

 sulphate, and therefore that the combination is not effected by 

 the equivalent substitution supposed. He mixed sulphate of 

 potash with H O, S O3 + H O, and found heat evolved, but 

 allows that the result here is fallacious, a portion only of the 



