V70 Dr, Thomson on the [March, 



aition of any crystals. But the moment we take out the stop- 

 per, the excess of sulphate of soda separates in a fibrous form, 

 so that the whole liquid assumes the appearance of an opaque 

 solid, while at the same time its temperature rises. I am not 

 aware that any person has hitherto attempted to give a satisfac- 

 tory explanation of this plienomenon. 



There are two salts which possess this property to a consider- 

 able extent; carbonate of soda, and sulphate of soda. Probably 

 there are more, but these are the two which I have examined 

 with attention. With sulphate of soda, the phenomenon never 

 fails ; but when we employ carbonate of soda, the success 

 depends entirely upon the temperature. If we can cool down 

 the solution below 50°, the success is certain; but at higher 

 temperatures than oO®, the crystals are not deposited imme- 

 diately, though they generally appear in a few hours. The 

 crystals formed are very different in these two liquids. In the 

 solutions of carbonate of soda, the crystals appear at the sur- 

 face in the form of small stars, not unlike flakes of snow. These 

 fall slowly through the liquid, giving the appearance of a shower 

 ■of crystals. The deposition goes on for some minutes, and the 

 crystals accumulate at the bottom of the phial, and at last fill it 

 for rather more than one-third of the portion occupied by the 

 liquid. The sulphate of soda begins likewise to crystallize at 

 the surface of the liquid ; but the crystals are so abundant, that 

 the whole surface becomes at once solid, and this crystallization 

 goes on slowly till it reaches the bottom of the phial in about a 

 quarter of a minute. The crystals thus formed put one very 

 much in mind of the fibrous variety of sulphate of lime. The 

 crystals gradually sink towards the bottom of the phial, and in 

 two or three days constitute a solid mass occupying at least 

 -four-fifths of the liquid, while the remaining fifth is a clear 

 transparent liquid occupying the upper part of the phial. 



To enable us to understand the nature of the phenomenon 

 more accurately, let us examine each of the two solutions a 

 tittle more closely. j 



1. Carbonate of soda is a salt composed of 



1 atom carbonic acid ... .^^^-^^^t. ^ 2*75 



1 atom soda = 4-0 



1 1 atoms water = 1 2*375 



ID- 125 



When heated, the water of crystallization is sufficient to cause 

 it to run into a liquid. When exposed to the temperature of 

 about 400°, it gradually loses the whole of its water, and is 

 converted into a hard, white, dry, saline mass, which dissolves 

 in water much more slowly than the crystals. It is scarcely 

 necessary to remark, that both the anhydrous and crystallized 

 salts are much more soluble in hot water than cold water. 



