Crystallization of Soda-alum. 27 



fectly bright, and not in the slightest degree effloresced ; but 

 the crystals had lost, from the escape of their hygrometric 

 water, that extreme and watery clearness which we have in 

 the crystal newly removed from its mother-liquor. From my 

 experience in respect to such salts, I had reason to believe 

 that the crystals of soda-alum were now in a most suitable 

 condition for analysis. 



Upon a very damp day the crystals were reduced to pow- 

 der and pressed in blotting-paper. A large crystal exposed 

 to the air at the same time lost nothing. 20*35 grs. of the salt 

 so prepared were exposed on a sand-bath to a heat, which was 

 gradually raised so as to effloresce the salt without melting it or 

 causing vesicular swelling. In eight hours the salt had been 

 heated above the melting point of tin, and had lost 8*98 grains. 

 It was thereafter heated, in a gradual and cautious manner, 

 to low redness by the spirit-lamp, and the loss became 9*65 

 grains. By a continued exposure to the same heat for half 

 an hour more, the salt lost only one hundredth of a grain ad- 

 ditional. Supposing it now to have lost all its water, the salt 

 will consist of 



Theory of 24 

 atoms of water. 

 Sulphate of alumina and soda 10-69 100' 100* 



Water 9-66 90-37 88-9 



20-35 J 90-37 188-9 



The calcined salt dissolved slowly but completely in boiling 

 water. By precipitation with muriate of barytes it afforded 

 21*22 grains sulphate of barytes, equivalent to 7-37 grains 

 sulphuric acid. Or the crystallized salt contains 34*73 per 

 cent, of sulphuric acid, while the theory of twenty-four atoms 

 of water supposes it to contain 34*93 per cent, sulphuric acid. 



It follows from this analysis that soda-alum contains twenty- 

 four atoms of water and not twenty-six. 



There is no reason to question the perfect accuracy of the 

 analysis of potash-alum by Berzelius, which gives to it like- 

 wise twenty-four atoms of water. Dried in the manner de- 

 scribed for soda-alum, I found it to consist of 



Theory of 24 

 atoms of water. 

 Sulphate of alumina and potash... 100* 100* 



Water 84*8 83*4 



184-8 183-4 



In such analyses, there is imminent danger of the water 

 carrying off a little acid with it, unless it is expelled in the 



E2 



