22 Mr. Graham on Exceptions to the Lata 



The precipitate by heat was exceedingly bulky and not 

 crystallized. It did not amount in general to so much as 3'8 

 grains from 8000 grains of the solution. Indeed the mean of 

 seven experiments made upon different solutions was 2'5 

 grains of precipitate. But it was found that the amount of 

 the precipitate depended much upon the time and agitation 

 employed in effecting the solution, as it is difficult to saturate 

 water with this salt. It is evident, therefore, that not the 

 mean but the greatest result will approach nearest to the 

 truth. 



Phosphate of magnesia boiled in water for several hours, 

 afterwards yielded solutions possessing this property. By the 

 heat, the crystals assumed the appearance in the water of 

 having effloresced. 



Phosphate of soda and sulphate of magnesia were added 

 separately to solutions of phosphate of magnesia, in the pro- 

 portion of 10 grains to 1000 grains solution, without influ- 

 encmg in the slightest degree the amount or appearance of 

 the precipitate. 



Phosphate of magnesia appears to be much more soluble 

 in the acids than in water : at least it was observed to dissolve 

 with facility in the following acids, even when in a very dilute 

 state — acetic, oxalic, phosphoric, muriatic, nitric, and sulphu- 

 ric. The addition of the smallest quantity of any of these 

 acids to the aqueous solution, prevents the appearance of the 

 usual precipitate by heat, by increasing the solvent power of 

 the menstruum*. 



In prosecuting this subject I had occasion to make several 

 observations. 



Mere continuance of the heat had no effect in increasing the 

 amount of the precipitate, in the solutions of hydrate of lime 

 or of phosphate of magnesia, provided no part of the solution 

 was at any time converted into vapour. When filtered solu- 

 tions of lime and phosphate of magnesia, which had formerly 

 been heated, were again subjected to a temperature of 212^ by 

 complete immersion in the water-bath in close vessels, and 

 retained at that temperature for several hours, no additional 

 precipitate appeared. But when heat of greater intensity was 

 applied to elevate the temperature of the solution to 212°, 

 this was seldom the case. When such a solution was heated 

 by the flame of a spivit-lamp, even in a close vessel, a slight 

 precipitate generally appeared. When the vessel, although 



* The experiment of the partial precipitation of phosphate of magnesia 

 in solution, by heat, has been rei)cated successfully by my friend Mr. A. 

 Steel, in the laboratory of Dr. Thomson, with great care and very pure 

 materials. 



close, 



