4G On the Influaice of the Pressure of the Atmosphere 



which is frequentlv equal to two centimetres of mercury, does 

 not produce any cd'cct, while a very small air-bell favours cry- 

 stallization so powerfullv ? In short, it is not less evident al- 

 ready, that tlie solvent power of water does not increase as the 

 jiressure of the atmosphere diininishes ; since a very small quan- 

 tity of any gas produces crystallization : but I am about to 

 show that it is entirely independent of it. 



If we introduce a crystal of sulphate of soda into a hyper- 

 saturated solution contained in a barometrical tube, crystalliza- 

 tion generally connnences, histantly extending speedily through 

 the whole mass, and the solution is afterwards at the same de- 

 gree of saturation which it would have acquired in the air at 

 the same temperature. Now I am convinced that crystals of a 

 salt introduced into its sohition, \vell saturated at a given tem- 

 perature, lower it but infrnitcly little under its true point of sa- 

 turation : consequently it is clear l.'eycuid dispute, that the sol- 

 vent power of water does not depend on the pressure of the at- 

 mosphere. We may add, that if we put into a tube a little salt 

 with its boiling and saturated solution, and afterwards make a 

 vacuum as described, a perceptible quantity of salt will not be 

 dissolved on applying heat. 



We shall see presently that the crystallization of a solution of 

 sulphate of soda may be prevented by allowing the latter to 

 cool in vacuo ; but we obtain the same result also by allowing 

 it to cool, exposed to atmospherical pressure, provided that we 

 cover the surface with a little spirits of turpentine. This me- 

 thod, which is also very efficacious for retarding the freezing of 

 water, is verV ea-.ily executed. Take a gla.^s tube from one to 

 two centimetres in diameter, closed at one end : pour in a boil- 

 ing and saturated solution of sulphate of soda, and cover it in- 

 stantly with spirit of turpentine. The solution will crystallize, 

 but rarely by cooling and even by agitation : a current of electric 

 fluid, or two platina wires communicating with the poles of a 

 pile, will produce no effect. But a crystal or a glass ring drop- 

 ped into it, will innuediately bring on crystallization: an iron 

 ling will do the same, but not so well as a glass one. 



if by diminishing the pressure of the atmosphere the solvent 

 power of water was increased, it would follow that, by augment- 

 ing the pressure at the surl'ace of a saturated solution of sul- 

 phate of soda, the ])recipitation of a part of the salt would be de- 

 termined; but this is not the case. I took a bent tube, the 

 large branch of which was two metres long ; and having intro- 

 duced into the shortest a solution of sulphate of soda, saturated 

 at the temperature of t'le air, I loaded it with a cohnunof mer- 

 cury two metres lii^jh, without any crystals being deposited even 

 after several davs. 



As 



