174 On the Deposition of Crystals hy Agitation. [March, 



a portion of clear liquid swims over them. Now if we examine 

 the crystals, we shall find that the lower part of them is still 

 fibrous ; but just under the hquid portion, there is a stratum of 

 regular prismatic crystals of sulphate of soda. It is obvious 

 from this, that the whole salt did not separate in crystals at 

 first. An additional quantity was obviously deposited after- 

 wards. Hence 1 overrated the weight of the salt deposited in a 

 fibrous state, and consequently the weight of its water of crystal- 

 lization. Nor is it difftcult to see the reason of this. The 

 increased temperature of the liquid (amounting to 24°) will of 

 course prevent the whole surplus salt from being deposited till 

 the liquid cools. I have not been able to determine the weight 

 of this second crop of crystals (as I did with respect to the solu- 

 tion of carbonate of soda); but from their appearance, they 

 cannot amount to a smaller proportion than one-fourth of the 

 whole mass of crystals deposited. Hence we have reason to 

 conclude, that the weight of the fibrous crystals at first deposited 

 was only 530 grains, instead of 713 grains. Now the water of 

 crystallization of this quantity of salt is about 300 grains, consti- 

 tuting ^ of the whole mass. Now 5*83 x 24 = 139-92. 



From this statement there seems no reason to doubt that the 

 water of crystallization of the salt which crystallizes gives out its 

 latent heat, and that this evolution is the cause of the augmen- 

 tation of temperature observed, though the difference of solubi- 

 lity has not been hitherto accurately determined. It will appear 

 from what follows, that at the temperature of 50°, 1 00 parts of 

 water dissolve about 14*5 parts of the dry salt, which is equiva- 

 lent to 48-01 parts of the salt in crystals ; while at the temper- 

 ature of 98°, 100 parts of water take up 23-69 parts of the dry 

 salt, equivalent to about 67*11 parts of the crystals. 



To form a solution of carbonate of soda which deposits crys- 

 tals when cooled down to 50° (on taking out the stopper) we 

 have only to dissolve an ounce troy of the dry salt in 4-22 ounces 

 of water. Now this is the same thing as dissolving 23*69 parts 

 of the dry salt in 100 parts of water. When the aljove solution 

 is cooled down to 50°, and the cork of the phial is drawn, a 

 copious precipitate of small crystals in stars takes place, and the 

 temperature of the solution is elevated, as nearly as I could 

 determine, 14°. 



