that Salts are more soluble in hoi than in cold Water. 23 



close, was only occupied in part by the solution, the precipi- 

 tate was greater ; and when the space occupied by the solu- 

 tion bore so small a proportion to the whole capacity of the 

 vessel, that the solution might be made to boil, and be con- 

 densed in the upper part of the vessel and returned without 

 loss, — the precipitate might be increased ad libitum, particu- 

 larly in the case of lime-water. The cause of the precipitate 

 appears to be the same in all these cases. The moment a 

 drop of the solution is converted into vapour, it deposits the 

 quantity of lime or salt that it held in solution ; and in the 

 case of bodies which dissolve so sparingly and with so much 

 difficulty, as the hydrate of lime and phosphate of magnesia, 

 although the water be returned again to the solution, it is in- 

 capable of re-dissolving what it has deposited. We know 

 that it would be a hopeless task to form a saturated solution 

 of lime, by agitating with the water no more than the few 

 grains which it is capable of dissolving ; and in the case be- 

 fore us, when the lime is once deposited the same difficulty 

 should be experienced in taking it up. 



These observations show the advantage of employing the 

 water-bath in heating the solutions, — a procedure which was 

 always followed by the author, and by which he regularly ob- 

 tained precipitates of hydrate of lime as well as of phosphate 

 of magnesia. They also account for a phenomenon in the 

 solubility of lime observed by Mr. Richard Phillips, which 

 otherwise appears anomalous*. 



Mr. Phillips heated a quantity of lime-water in a flask, the 

 neck of which was elongated by a tube, to prevent the access 

 of carbonic acid gas from the atmosphere, and made to boil 

 till 1-1 3th part was dissipated in vapour. From the deposi- 

 tion which mereelevationof temperature would occasion with- 

 out any evaporation, the quantity of lime in solution would be 

 reduced to x^Vo^'^ part, but it was found to amount to no 

 more than yjij y. But much more of the solution would be 

 converted into vapour during the boiling, than what actually 

 escaped, the cool sides of the long tube being singularly adapt- 

 ed to condense the rising vapour and return it to the solution, 

 supposing that the tube had any elevation ; while the hydrate 

 of hme, which had been deposited in hard crystals, would not 

 admit of being re-dissolved in an appretiable degree. 



It is evident that this effiict of cohobation will take place, 

 not only in lime-water and the solution of phosphate of mag- 

 nesia, l)ut to a certain extent in all bodies of difficidl solubi- 

 lity. I have observed it to u considerable extent in the solu- 

 tion of sulphate of lime, even when greatly diluted, and be- 



♦ Annals of Philosophy, N. S. vol i. p. lO't. 



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