SCIENTIFIC RECREATIONS. 



of copper, or chromate of potash, are coloured ; others, like sulphate of soda, 

 are colourless. The last-mentioned salt, with a number of others, will take 

 a crystalline form ; if dissolved in boiling water, and the solution left to 

 stand, we shall perceive a deposit of transparent prisms of very remarkable 

 appearance. This was discovered by Glauber, and was formerly called 

 Glauber's salts. 



Sulphate of soda is very soluble in water, and at a temperature of thirty- 

 three (Centigr.) water can dissolve it in the greatest degree. If we pour a 

 layer of oil on a solution saturated with Glanber's salts, and let it stand, it 

 will not produce crystals ; but if we thrust a glass rod through the oil into 

 contact with the solution, crystallization will be instantaneous. This singular 

 phenomenon becomes even more striking when we put the warm concentrated 

 solution into a slender glass tube, A B, which we close after having driven 



Fig. 306. Preparation of a solution saturated with sulphate of soda. 



out the air by the bubbling of the liquid (fig. 306). When the tube has 

 been closed, the crystals of sulphate of soda will not form, even with the 

 temperature at zero ; nevertheless the salts, being less soluble cold than hot, 

 are found in the fluid in a proportion ten times larger than they would 

 contain under ordinary conditions. If the end of the tube be broken the 

 salt will crystallize immediately. We will describe another experiment, but 

 little known and very remarkable, which exhibits in a striking manner the 

 process of instantaneous crystallizations. Let one hundred and fifty parts 

 of hyposulphite of soda be dissolved in fifteen parts of water, and the solu- 

 tion slowly poured into a test-glass, previously warmed by means of boiling 

 water, until the vessel is about half-full. One hundred parts of acetate of 

 soda is then dissolved in fifteen parts of water, and poured slowly into the 

 first solution, so that they form two layers perfectly distinct from each other. 

 The two solutions are then covered with a little boiling water, which, how- 

 ever, is not represented in our illustration. After it has been left to stand 



