Dr Ogden (m Saline Crystallization. 311 



three or four different salts, shewing that the property was not 

 pecuHar to tlie sulphate of* soda, 



u,In the Repertory of Arts for 1818, vol. xxxiii, is a paper by 

 Dr Ure, in which are detailed some curious experiments, tend- 

 ing to shew that negative electricity has a powerful effect in 

 promoting crystallization. He operated on sulphate of soda, 

 and also arrives at the conclusion that " neither the chemical 

 properties of the atmosphere, nor its pressure, have any influ- 

 ence on crystallization." 



Since then, Mr Graham of Glasgow has rendered it probable 

 that the chemical properties of the atmosphere, or at least of 

 some gases, are capable of inducing crystallization. His expe- 

 riments consisted in throwing up small quantities of gases 

 through mercury into supersaturated solutions of sulphate of 

 soda, and he concludes that " air determines the crystallization 

 of supersaturated saline solutions, by dissolving in the water, 

 and thereby giving a shock to the feeble power by which the 

 excess of salt is held in solution.'" Ammoniacal gas was found 

 most efficient. The experiments are detailed in the Transac- 

 tions of the Royal Society of Edinburgh for 1828. They also 

 prove crystallization to be independent of the pressure of the 

 atmosphere. 



Notwithstanding these authorities, the doctrine of atmosphe- 

 ric pressure occasioning crystallization is yet very generally re- 

 ceived, and may be traced in some of our best works on chemis- 

 try. There is a certain appearance of analogy between the con- 

 version of elastic gas into a liquid by the force of strong pressure, 

 and the conversion of a supersaturated liquid into a crystalline 

 solid by the same means, which has greatly tended to give cur- 

 rency and plausibility to the latter doctrine. But the analogy is 

 deficient in one very important particular. The liquefaction of 

 a gas is attended with an immense reduction of volume; whilst in 

 the solidification of a liquid, the volume undergoes little or no 

 change. There are also circumstances in the mode of experi- 

 menting which may have been instrumental in perpetuating the 

 e^!for^ .When the cork or stopper is removed from a closed 

 yessel containing a supersaturated solution, it is very liable to 

 occasion a few almost imperceptible fragments of salt to be de- 

 tached from the neck, which, falling into the liquid below, inj. 



