Dr Ogden on Saline Crystullizaiion. 3 IS 



These experiments clearly demonstrate that atmospheric pres- 

 sure alone does not produce crystallization. 



I Owing to its remarkable property of being most soluble at 

 the temperature of 106® Fahrenheit, sulphate of soda is particu- 

 larly well adapted for these illustrations. It does not require so 

 gradual and careful a reduction of temperature as other salts; 

 the solution may be filtered at any high temperature near the 

 boiling point, for it has no tendency to crystallize until it has 

 fallen to 106°. With many salts, crystallization commences 

 with the least reduction of temperature, and if the attempt is 

 made to filter their solutions fully saturated, they immediate- 

 ly begin to crystallize. It is therefore necessary to use them 

 somewhat under saturation at the boiling heat. 



4. A strong solution of muriate of lime was closed up whilst 

 hot, and allowed to cool. It was several times freely agitated, 

 without producing any effect. On exposing the phial contain- 

 ing it to a blast of cold wind, it crystallized. The same effect 

 may be produced by dashing cold water on the outside of a ves- 

 sel containing a supersaturated solution. 



The seclusion from atmospheric pressure in this case had no 

 effect in preventing crystallization. 



5. To ascertain if a supersaturated solution was capable of 

 exhibiting its proper chemical qualities without crystallizing, I 

 again heated and gradually cooled the vessel containing muriate 

 of lime. Into the cold supersaturated solution thus formed, I 

 let fall two drops of concentrated sulphuric acid, which pro- 

 duced a solid crust of sulphate of lime, and evolved fumes of 

 muriatic acid, but did not induce crystallization. 



6. The last experiment was repeated, shaking the mixture at 

 the time the sulphuric acid was added. It did not, however, 

 crystallize 



7. Decomposition was likewise effected by adding oxalate of 

 ammonia to the same solution. No crystallization was pro- 

 duced. 



\ 8. Nitrate of bary tes was dropped into a supersaturated solu- 

 tion of sulphate of magnesia, forming sulphate of barytes and 

 nitrate of magnesia. The solution did not crystallize. 



The only result of the four last experiments is, to shew that 



