125 
REPORT ON THE DRIED ALUM ” OP COMMERCE. 
It is from the ammoniacal liquors of the gas-works that the alkali is ob¬ 
tained which now enters into the composition of alum ; ammonia from this 
source is cheaper than potash, and answers here equally well: for the uses 
to which alum is put in the arts being solely for the sake of the alumina, it is 
obvious that the alkali present is comparatively immaterial. But even 
commercial ammonia alum is never perfectly pure ; that is, it cannot be said 
to have the formula ALOg, SSOg -f NH 4 O, SO 3 + 24 HO, which would 
give it a percentage composition of— 
AhOg, SSOg = 37-78 
NH^O, S03= 14-56 
24 HO =47-66 
100-00 
For potash derived from natural sources is, in small quantity, nearly always 
present, amounting generally to about one or two per cent. ; this is a well- 
known fact, and is proved also by what follows. The ammonia was estimated 
here, as well as in the subsequent analyses, by combustion with soda lime, 
and is shown as sulphate. 
Sulphate of ammonia. Theory. 
A. 1-1355 grm. gave -136 = 1T97 x^er cent. . . 14-56 
B. 1-4155 „ ^ „ -169 = 1T23 „ „ . . 14-56 
These two samples were quite distinct. 
“ Potash ” is easily recognized in a specimen, by adding chloride of ammo¬ 
nium and caustic ammonia to the solution of the alum in water, filtering off 
the p)i’ecipitated alumina, evaporating to dryness, and igniting to get rid of 
ammoniacal salts ; if there is any residue, it must be potash: by dissolving 
this in water and adding perchloride of ^datinum, its presence or absence is 
immediately ascertained. Dried alum then is, of course, always made from 
the ammonia variety, and five specimens were procured from different houses 
in order to ascertain the comi)osition of the commercial article; they were 
quantitatively examined for ammonia and water, and qualitatively for potash : 
the results are given in percentages. 
no 
NH, 0 ,S 03 
K 0 ,S 03 
Remarks. 
I. 
16-35 . 
. 14-07 . 
. Present . 
. Completely soluble in 
water, no ox)acity. 
II. 
38-92 . 
. None . 
. None 
. Scarcely soluble in 
water all. 
III. 
45-51 . 
. 10-57 . 
. Present . 
. Partially soluble, solu¬ 
tion presenting an 
opaque appearance. 
IV. 
37*86 . 
. 6-29 . 
. Present . 
. Solubility exceedingly 
slight. 
V. 
43-05 . 
. 12-57 . 
. Present . 
. Completely soluble, no 
opacity. 
From this we see that the proportion of ammonia varies considerably in dif¬ 
ferent specimens; in one case only does it nearly come U]d to the coiyect 
number, viz. 14’56 per cent., and as I happened to ascertain the history of 
that particular sample, I conclude, by the manner in which it was exsiccated, 
that the remaining 16 per cent, of water had never been driven off, for, having 
been kept in a stoppered bottle and rarely opened, it could not have re¬ 
absorbed the water from the atmosphere, as the other specimens had done; 
