1822.] peculiar Sulphate of Alumina. 281 
quantity was obtained; indeed [ found that during several 
months, the solution continued depositing, but the substance 
had not in any degree a crystalline form. Another property of 
this solution is worthy of notice : if some of it be put into a tube, 
and placed in water of the temperature of 160° to 170°, and 
probably even lower, it becomes opaque and thick in a few 
seconds ; if, however, the tube and its contents be kept at the 
ordinary temperature of the air for several days, the precipitate 
is gradually redissolved, and the solution regains its transpa- 
rency. It appears extremely singular that this solution should 
have required so long a time for its production, and perhaps still 
more so that the peculiar sulphate of alumina in question was 
not deposited as quickly as it was formed ; yet I did not observe 
any disposition to deposit until after the removal of the excess 
of alumina floating in the solution. Not anticipating the spon- 
taneous deposition which I have described, I did not take the 
specific gravity of the solution at its greatest density ; but after 
it had continued depositing for several weeks, I found the spe- 
cific gravity of the solution exceeded 1-120. 
Although the solution of sulphate of alumina continued 
affording a deposit for many months, yet it did not appear to 
suffer any change of composition, for water added to it at this 
period continued to occasion precipitation, which it probably 
would not have done, if the deposit consisted of alumina com- 
bined with less acid than when in solution, for the excess of acid 
which must have remained in solution, would probably have pre- 
vented the precipitating action of the water. 
As metallic oxides which are precipitated from acid solutions 
by water, usually contain a portion of the acid which held them 
in solution, there could be no doubt that the precipitate formed 
in this sulphate of alumina by water was a subsulphate, and I 
found it to be so, but I have not yet had leisure to determine its 
composition. 
It is well known that it is extremely difficult to deprive alum 
of the whole of its sulphuric acid, and [ found that alumina, even 
when precipitated from solution by excess of ammonia, and 
ignited, gave a precipitate with muriate of barytes when redis- 
solved in an acid. It appeared to me, therefore, a question to 
be decided what quantity remains in combination with the alu- 
mina. I dissolved 1000 grains of alum in water, precipitated the 
alumina by carbonate of soda, and washed it with distilled water 
until it ceased to afford sulphuric acid, as determined by nitrate 
of barytes. I then dissolved the alumina in nitric acid, and 
added nitrate of barytes as long as precipitation occurred ; the 
sulphate of barytes when dried weighed 24 grains, consequently 
the precipitated alumina contained 8-1 of sulphuric acid ; and as 
1000 of alum yield about 110 of alumina, I shall, in the experi- 
ments which | am going to state, deduct 7°36 per cent. from the 
precipitates of alumina, considering it as sulphuric acid. 
