316 Composition of Crystals of Sulphate of Soda. 



not formed ; but in the fiist half of the third day (during the 

 two first days, from seven o'clock in the morning till nine 

 o'clock in tlie evening), fire had been kept in the furnace, and 

 its formation could not longer be perceived than during about 

 half an hour*. These experiments leave, I think, no doubt 

 that the solid acid really was anhydi'ous sulphuric acid. Its 

 formation may thus be explained,— that, in a certain concen- 

 tration of the aqueous sulphuric acid, part of the acid yields 

 its water to another part of the acid, and is voladlized ; where- 

 by, on one side, by the great volatility of the anhydrous acid, 

 on the other side by the great fixity of the acid containing 

 water, this kind of decomposition seems to be induced. — 

 BrCiXsto's Juiirti. 



MR. FARADAY ON THE COMPOSITION OF CRYSTALS OF SUL- 

 PHATE OF SODA. 



It is known that when a hot strong solution of sulphate of 

 soda is put into a vessel and closed up, it may be reduced to 

 common temperatures without crystallizing, although, if the 

 vessel be opened, abundance of crystals will immediately form. 

 It has also frequently been observed that, in some circum- 

 stances, crystals would form in the solution during cooling, 

 even though the vessel had not been opened or agitated. 

 These ci'ystals, when observed in the solution, are very trans- 

 parent and of a large size; they are quadrangular prisms, 

 with diedral summits. Upon opening the vessel, the sur- 

 rounding solution crystallizes rapidly, enveloping the first 

 formed set of crystals with others, which, however, are very 

 readily distinguished from them in consequence of their im- 

 mediately assuming a white opaque appearance. Upon taking 

 out the crystals, those first formed are found to be much harder 

 than the usual crystals of sulphate of soda; and when broken, 

 it is found that the opacity is not merely superficial, but that 

 it penetrates them to a considerable depth, and even at times 

 throughout. 



These harder and peculiar crystals are readily obtained by 

 closing up a solution of sulphate of soda, saturated at 180°, in 

 a Florence flask, boiling the solution in the flask so as to expel 

 the air before closing it. Upon standing 24 hours, fine groups 

 of crystals arc formed. When the flask is opened, the solution 

 deposits fresh crystals ; but on breakhig the flask, the latter 

 may be scraped off" by a knife, in consequence of the superior 

 hardness of the first set. 



* I quote these circmnstances, that it may appear how slowly the dis- 

 tillation piocecdeii. Probably no fuming acid will be formed when the fluid 

 in the retort is brought to boiling. 



I'he 



