FOREIGN CORRESPONDENCE. 



399 



solution of common salt (chloride of sodium) and, after having slightly 

 dried it, heats it to a good red heat in a platinum crucible. Soon 

 vapours of hydro-chloride acid are given off (showing that the salt is 

 decomposed) and silicate of soda is found to have formed in the kaolin. 

 This experiment is repeated three, four, or five times, until the kaolin 

 DO longer decomposes the salt, when the earth is seen to have hecome a 

 granular mass of crystals, easily fusible, and so hard as to scratch glass 

 with facility. The same experiment was tried with chloride of calcium 

 and chloride of magnesium instead of salt, and with similar results. 

 Chloride of magnesium and proto-chloride of iron act with far greater 

 ease than common salt. But, as a remarkable exception, chloride of 

 potasium gives no result at all. This becomes an extremely interesting 

 fact, since M. Delesse has shown formerly that those feldspars, which 

 seem to have been produced by metamorphism, are never found to 

 contain potassa. 



Sandstones. — One experiment is alone recorded : — The author took a 

 piece of sandstone, essentially quartzose, and containing little or no ad- 

 mixture whatever — it was the grh (V Orsay, with which the streets of 

 Paris are paved. He soaked it alternately with a solution of chloride 

 of magnesium and a solution of chloride of calcium, and then submitted 

 it to a red heat. Aftei^ a certain number of successive imbibitions and 

 calcinations, the sandstone was observed to become spungy ; it absorbed, 

 without difficulty, a considerable quantity of liquid, and was easily 

 pulverized in an agate-mortar. Thus pulverized, it was heated in a 

 crucible to a white heat, when it melted, and was transformed into a 

 milky mass, composed of numerous long crystalline needles. It had a 

 specific gravity of 3 00, was insoluble in acids, and did not contain a 

 trace of chlorine. "When analyzed, it gave the composition of amphi- 

 bolite or pyroxene, of which it showed the specific gravity. 



Hydrophane is opal (hydrated silica), which has the remarkable 

 property of becoming transparent or translucid when placed in water, 

 and losing its transparence again when taken out of the liquid. M. 

 Langlois^'' has just produced artificial hydrophane in the following 



* We remember having read in the Comptes Rendus, of the Paris Academy of 

 Sciences, a memoir published many years ago by Ebehnan, late chemist of the 

 porcelain manufactory of Sevres, where he describes a process by which he produced 

 hyalite ; and, if we are not mistaken, hydrophane also. We will turn again to 

 this memoir at the first opportunity. — T. L. P. 



