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the potash also in it was very minute, l^o. II. consisted entirely of basic sul¬ 
phate of alumina, and was insoluble in water. 
. These results are, I think, about what one would expect when operating 
on ammonia alum. According as the heat used be more or less intense or 
long continued, so larger or smaller quantities of sulphate of ammonia will be 
volatilized, and one would not be surprised to find a carelessly prepared 
specimen consisting of little else than alumina. 
Acting upon these considerations, I endeavoured to dry with the utmost 
care the sample previousl}?^ noticed as “A,” so as to see how much ammonia 
I could possibly retain, and yet deprive the alum of the whole of its water. 
It was therefore dried in an oil bath till it ceased to lose weight, and then 
carefully heated over an, air flame in a platinum crucible ; keeping it con¬ 
tinually stirred, so as to prevent the lower portions becoming more heated 
than the upper layers. When it had lost 47|- per cent, in weight the residual 
ammonia was determined :— 
After drying. Before drying. 
A. 2'4 grm. gave lOT . . . 11*97 per cent. 
It had lost T87 per cent, of sulphate of ammonia. 
In the previous analyses each specimen will be noficed to contain a large 
proportion of water. Now I do not in any case, except that of No. I., sup¬ 
pose this to be water of crystallization, but moisture which has been reab¬ 
sorbed subsequent to exsiccation. Alum in this state is exceedingly hygro¬ 
scopic. 
Bliicher mentions that “ burnt alum exposed to the air in summer absorbs 
18 atoms of water in 47 days, and the absorption does not cease even then.” 
This also tends to upset the idea that, weight for weight, dried alum is much 
stronger than the crystalline. Soon after drying this may be the case, but 
in most instances they will be found of about the same strength. 
The United States Pharmacopoeia orders potash alum, but foreseeing that 
ammonia alum would be used, orders 450° P. to be the highest heat em¬ 
ployed, and checks the operation when nearly all the water has been vola¬ 
tilized, as indicated by the weight of the residue. If potash alum be used, 
474*5 grains should yield 258 of dry salt; that is, 4 per cent of water is to be 
left behind. This is evidently the proper way to prepare it: it prevents the 
loss of sulphate of ammonia, as well as of sulphuric acid from the alumina, 
and ensures at the same time uniformity of product, and solubility in water. 
Burnt alum takes a long time to dissolve in water; in fact, when freshly 
prepared and anhydrous it is insoluble. Alum thrown into water immediately 
after it has been burned and become cool, remains almost entirdy undissolved 
after many months ; but if it has been previously exposed to the air for several 
days, it regains its peculiar taste and dissolves with ease. 
We have seen, then, that dried alum always is, and probably now always 
will be, made ifrom ammonia alum (the only potash alum now procurable 
being an entirely natural product obtained in the vicinity of Borne) ; that 
unless very great care be used in the application of heat, which should not 
exceed 450°, no two products will be alike ; and further, that owing to the 
rapid re-absorption of water after preparation, it scarcely seems worth the 
trouble of previously exsiccating it. In fact, the only advantage that I can 
see possessed by dried alum over the crystalline variety, is that in its limited 
application as an escharotic to fungous ulcers it is in an amorphous state, and 
possibly less likely to irritate than angular fragments of crystals. It seems 
absurd to drive off the water and then to redissolve it again in -water for use 
in lotions or similar applications. 
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