i72 Dr. Thomson on the [March, 



deal of the liquid within their interstices. This liquid I endea- 

 voured to get rid of by means of filtering paper, which imbibed 

 it ; but after the crystals were rendered as dry as possible by 

 this means, they were still far from being perfectly so. A small 

 portion of liquid must still have been contained within the inter- 

 stices of the Crystals. Now this liquid still held a consi- 

 derable quantity of carbonate of soda in solution. Hence the 

 weight of anhydrous carbonate which I obtained must hav« 

 somewhat exceeded the truth. Had the quantity been such that 

 the water of crystallization which it contained amounted to 



~ instead of -^^^ then the elevation of temperature observed 



would have been exactly equal to the latent heat. 



I have no doubt that these two sources of error taken toge- 

 ther are the cause of the small difference of half a degree 

 between the theoretical and practical results. 



When the liquid from which the crystals of carbonate of soda 

 had been deposited was set aside for two or three days, an addi- 

 tional crop of crystals separated from it. These crystals were 

 weighed, and found to amount to 214-6 grains, which is equiva- 

 lent to 75-89 grains of the dry salt. Thus it appears that 8-13ths 

 of the surplus salt are deposited immediately in crystals, while 

 the remaining 5-13ths remain in solution ; but are notwith- 

 standing deposited in the course of two or three days in the 

 state of crystals. Thus the liquid was at last reduced to the 

 state of a saturated solution at 50°. 



2. Sulphate of soda is a salt composed of 



1 atom sulphuric acid = 5*0 



1 atom soda 4*0 



10 atoms water 1 1'25 



20-25 



When heated moderately, its water of crystallization is suffi- 

 cient to cause it to liquefy. Gay-Lussac has shown that water 

 of the temperature 106*^ dissolves a maximum of this salt, and 

 that the solubility diminishes when the temperature is increased. 

 I have reasons for believing that carbonate of soda is distin- 

 guished by a similar property, but its maximum point of solubi- 

 lity is as high as 120°. At that temperature water is capable of 

 taking up a greater quantity of the anhydrous carbonate than at 



To form a solution of sulphate of soda capable of crystallizing 

 when agitated, we have only to dissolve 51 parts of the crystal- 

 lized salt in 49 parts of water ; or, which is the same thing, 

 22-44 parts of the anhydrous salt in 77*56 parts of water ; or 

 28*91 parts of the anhydrous salt in 100 parts of water. This 

 constitutes a saturated^ solution at the temperature of 88*25^. 



