24< Mr. Graham oti Exceptions to the La'w 



Ueve that the deposit from slight boiling observed in many 

 mineral waters, and generally attributed to the dissipation of 

 carbonic acid gas, depends in some instances upon this cause. 

 However weak the solution may be, it is evident that a por- 

 tion of the salt may be deposited in this way. 



It had occurred to us as a method of determining the rela- 

 tive solubility at different temperatures of bodies of this class, 

 to form a saturated solution at the lowest temperature, and 

 dilute it with water till it ceased to deposit at the high tempe- 

 rature. But this method was found inconvenient from the 

 difficulty of incorporating the solution with the water added. 



4000 grains lime-water were diluted with 2000 grains water, 

 agitated and set aside for two hours. Upon being then heated 

 in the water-bath to 212°, a precipitate appeared, which being 

 received upon a filter and dried was found to amount to nearly 

 2 grains hydrate of lime. Phosphate of magnesia similarly 

 treated gave 12 grains of precipitate. 



4000 grains lime-water diluted with an equal quantity of 

 pure water, and occasionally agitated for three days in a stop- 

 pered phial, became slightly turbid upon being carefully heat- 

 ed in the water-bath, and deposited a small quantity of hy- 

 drate of lime, of which 0'15 grain was recovered. The so- 

 lution of phosphate of magnesia in the same circumstances 

 yielded a precipitate, which although it rendered the solu- 

 tion much more turbid, did not amount to so much. 



It was found, as might be expected from the previous ex- 

 periments, that the deposit by heat from lime-water was not 

 diminished sensibly by being allowed to remain in the solu- 

 tion till it became cool, or was not re-dissolved upon cooling. 

 Hence it is unnecessary to filter the solution while hot. The 

 phosphate of magnesia, however, appeared to be re-dissolved 

 in a more sensible degree, probably from the state of extreme 

 division in which it is deposited. At least 2*3 grains of pre- 

 cipitated phosphate of magnesia were obtained by filtering at 

 212'', while an equal quantity of the same solution, allowed to 

 cool with occasional agitation before filtration, gave a preci- 

 pitate which did not exceed 2 grains. In appearance, the pre- 

 cipitate had suffered a very great reduction. 



The rapidity with which phosphate of magnesia effloresces 

 when exposed to the atmosphere, led us, from theoretical con- 

 siderations, to look for this anomaly in its solubility. Efflo- 

 rescence in the hydrates of the salts certainly indicates a weak 

 affinity for water at the atmospheric temperature — an attraction 

 or affinity, too, which is much diminished by slight elevation 

 in temperature. If the attraction subsisting between the salt 

 and water, when in solution, be of the snmc nature as that be- 

 tween 



