the Crystallization of Saline Solutions. 217 



the solution around the bubble. A slight reduction in the 

 solvent power of the menstruum will ensue at the spot where 

 the air is dissolved. But the menstruum is greatly overloaded 

 with saline matter, and ready to deposit ; the slightest diminu- 

 tion of its solvent power may therefore decide the precipitation 

 or crystallization of the unnatural excess of saline matter. The 

 absorption of air may in this way commence and determine the 

 precipitation of the excess of sulphate of soda in solution. 



Here, too, we have an explanation of the fact just mentioned, 

 that solutions of sulphate of soda which have not been boiled, 

 are less affected by exposure to the air than well-boiled solu- 

 tions ; for the former still retain the most of their air, and do 

 not absorb air so eagerly on exposure as solutions which have 

 been boiled. 



But the theory was most powerfully confirmed by an expe- 

 rimental examination of the influence of other gases, besides 

 atmospheric air, in determining crystallization. Their in- 

 fluence was found to be precisely proportionate to the degree in 

 which they are absorbed or dissolved by water and the saline 

 solutions. 



To a solution of sulphate of soda over mercury, which had 

 not been affected by a bubble of atmospheric air, a bubble of 

 carbonic acid gas was added. Crystallization was instantly 

 determined around the bubble, and thence through the whole 

 mass. Water is capable of dissolving its own volume of car- 

 bonic acid gas, and a solution of sulphate of soda as strong as 

 could be employed was found by Saussure to absorb more 

 than half its volume. 



In a solution of sulphate of soda, which was rather weak, 

 both common air and carbonic acid gas failed to destroy the 

 equilibrium ; but a small bubble of ammoniacal gas instantly 

 determined crystallization. 



When gases are employed which water dissolves abundant- 

 ly, such as ammoniacal and sulphurous acid gases, the cry- 

 stallization proceeds most vigorously. It is not deferred till 

 the bubble of gas reaches the top of the receiver, as always 

 happens with common air, and frequently with carbonic acid 

 gas, but the track of the bubble becomes the common axis of 

 innumerable crystalline planes, upon which it appears to be 

 borne upwards ; and sometimes before the ascent is completed, 

 the bubble is entangled and arrested by crystalline arrange- 

 ments which precede it. 



The number of gases which are less soluble in water than 

 atmospheric air is not considerable ; but of these, hydrogen gas 

 was found to be decidedly least influential in determining cry- 

 stallization. 



New Series. Vol. 4. No. 21. Sept. 1828. 2 F Minute 



