■44 On the Infiuence of the Pressure of the Atmosphere 



which favour its solution when we treat the crude platina with 

 the nitro-muriatic acid. For the same reason, it was impossible 

 to study the properties of the simple salts of this metal. 



I shall give in a subsequent memoir the method of separating 

 the osmium and iridium from the foreign bodies which foruj 

 the insoluble residue of the ore of platina, and I shall explain 

 in the same way the properties which these two metals shall ex- 

 hibit on examination. 



X. On the Infliievce of the Pressure of the Atmosphere on the 

 Crystallization of Salts. By Gay Lussac*. 



Oeveral chemists have laid it down as a principle, that we in- 

 crease the solvent jjower of water by diminishing atmospherical 

 pressure. This result is not however well ascertained ; for we 

 are acquainted with only one substance, the sulphate of soda, 

 th* aqueous solution of which does not crystallize in vacuo, 

 although it gives abundance of crystals in the open air ; and be- 

 sides, the circumstances under which this pliaenomenon takes 

 place have not been accurately noted. I propose therefore to 

 examine, 1. The causes which concur in the crystallization of 

 the sulphate of soda when we vary the pressure of the atmo- 

 sphere; and, 2. What other substances which are soluble in 

 water, act in the same manner. 



In order to make the experiments which I am about to de- 

 scribe, a common barometrical tube may be used, into which 

 the saline solutions are to be introduced when warm : this may 

 be filled about three-fourths, and in order to form a vacuum, the 

 solution must be boiled. When it is juesumed that the vapour 

 has completely displaced the air, the open end of the tube must 

 be hermetically sealed, by exposing it to the blow-pipe, or by 

 dipping it in melted sealing-wax. The vacuum obtained in this 

 way may be regarded as perfect nearly to the degree of the 

 elastic force of the vapour of the saline solution. 



The temperature of the air being from 12° to 18°, I in- 

 closed a solution of sulphate of soda at the boiling temperature, 

 in a small tube as above described. After cooling, no crvstal 

 was formed, although it was shaken frequently. Having broken 

 the extremity of the point of the tube to give access to the air, 

 the liquid instantly became solid, with a very perceptible dis^ 

 engagement of heat. It happens sometimes, however, that the 

 entrance of the air does not determine the crystallization ; but 

 if we then introduce a small crystal into the solution, or if we 

 thake it, it begins to crystallize immediately. This circum- 



* Memoires d'Arcncil, tome- iii. October 1813. 



Stance 



