Mr GRAHAM on the Crystallization of Saline Solutions. 115 



verted over mercury in the pneumatic trough, and still remain 

 liquid on cooling ; and thus the causes which determine crystal- 

 lization were more readily examined. For this purpose, it was 

 absolutely necessary that the mercury in the trough should be 

 previously heated to 110 or 120 ; for otherwise that part of the 

 solution in contact with the mercury cooled so rapidly, as to deter- 

 mine crystallization in the lower part of the receiver long before 

 the upper part had fallen to the temperature of the atmosphere. 

 In such cases, crystallization beginning on the surface of the 

 mercury, advanced slowly and regularly through the solution. 

 Above, there always remained a portion of the solution too weak 

 to crystallize, being impoverished by the dense formation of cry^ 

 tals below. It was also necessary to clean the lower and exter- 

 nal part of the receivers, when placed in the trough, from any 

 adhering solution, as a communication of saline matter was some- 

 times formed between the solution in the receiver and the atmo- 

 sphere without. When these precautions were attended to, sa- 

 line solutions over mercury remained as long without crystallizing 

 as when separated from the atmosphere in the usual mode. 



Solutions which completely filled the receivers when placed 

 in the trough, allowed a portion of mercury to enter, by con-, 

 tracting materially as they cooled. A bubble of air could thus 

 be thrown up, without expelling any of the solution from the 

 receiver, and the crystallization determined, without exposing 

 the solution directly to the atmosphere. 



The first observation made was, that solutions of sulphate of 

 soda sometimes did not crystallize at all upon the introduction 

 of a bubble of air, or at least for a considerable time. This irre- 

 gularity was chiefly observed in solutions formed at temperatures 

 not exceeding 150 or 170, although water dissolves more of the 

 sulphate of soda at these inferior temperatures than at a boiling 

 heat. Brisk ebullition for a few seconds, however, rendered the 

 solution upon cooling amenable to the usual influence of the air 



