1822.] peculiar Sulphate of Alumina. 283 
muriatic acid were decomposed by carbonate of soda, and 28°8 
of alumina remained after ignition. Deducting 7:36 per cent. 
from the alumina for sulphuric acid, this deposited sulphate of 
alumina very closely resembles that of the solution, and appears, 
therefore, to have been deposited without any decomposition. 
I have just shown that the solution consists of 
Rad Ns Nahe nal pinta ae wne «i arate 40-0 
Allpiairtin gs! hi, i] GS Sele ee ath ele . 40°83 
while the deposited sulphate is composed of 
Sulpimnricdevdy le Sse eH Salt ». 40°0 
ADDED sh oc hegs sioe4 dre onshore dips, ofage OTe 
From various considerations, more especially the constitution 
of alum, I am induced to differ from Dr. Thomson as to the 
weight of an atom of alumina. I shall take an early opportunity 
of returning to the subject ; and, I think, I shall be able to show, 
contrary to his views, that alum contains a supersalt. Among 
other reasons for this opinion, I may state one experiment which 
I have very frequently repeated. If zinc filings be added to a 
solution of alum, they are gradually dissolved, but with suf- 
ficient rapidity to give out enough hydrogen gas to cause an 
explosion when a flame is presented. 
According to my present opinion, an atom of alumina weighs 
27, or one-half more than determined by Dr. Thomson. On 
this view the deposited sulphate of alumina which I have 
described will consist of 
By theory. By exper. 
2 atoms of sulphuric acid 40 x 2 = 80 ...... 80°00 
3 atoms of alumina 27 x 3... = 81 ..... » 876 
I have already observed that when this solution is mixed with 
water, it is decomposed; and I have some reason for believing 
that the sulphate of alumina which remains in solution is that 
which with bisulphate of potash forms alum, the precipitate 
being, as I have ascertained, and indeed already mentioned, @ 
subsulphate of alumina. 
