48 PROFESSOR GREGORY ON THE EXTRACTION OF i 
shew that the platinum has been strongly corroded. In short, this method, save 
on a very small scale, is so objectionable, that it is seldom employed except for 
illustration. 
The second process is one recently proposed, I believe, by Lrepic, founded on 
the fact, that the whole lime may be removed from the superphosphate of lime, 
by the addition of sulphuric acid to the concentrated solution. This causes it to 
become quite thick from the large quantity of sulphate of lime produced. Cold 
water being added, the whole is filtered, and the filtered liquid and washings 
again concentrated and filtered from any sulphate of lime deposited during the eva- 
poration. The concentrated liquid is again tested for lime by sulphuric acid, and 
if no change ensues, the lime has been entirely removed, as I ascertained by the 
proper tests. The solution now contains only the whole phosphoric acid of the 
bones, the magnesia which they always contain, and more or less free sulphuric 
acid. I have described this process thus far minutely, because, up to this point, 
it is the same as that which I recommend; and it is in the mode of separating 
the magnesia that the advantage of my process consists, 
Liesie acts on the concentrated acid solution, brought to the consistence of 
syrup, by alcohol, which dissolves the phosphoric acid, leaving undissolved the 
greater part of the phosphate of magnesia, and depositing the last traces when 
allowed to stand. I cannot ascertain whether this operation is to be performed be- 
fore or after the sulphuric acid has been expelled by heat; but I find, that after 
expelling the sulphuric acid, a transparent and colourless glass is obtained, which 
dissolves perfectly in boiling water, and the solution concentrated to a syrup, and 
treated with alcohol, yields a solution containing much magnesia, and which, on 
standing for weeks, deposits nothing. It is very probable that the alcohol did 
not succeed in separating the magnesia in my experiments, because the phos- 
phoric acid was in some one of its modifications, different from that in which 
Liebic employed the same method. But I have not yet been able to manage 
that process so as to answer the purpose intended ; and, even if it did succeed 
better, it is well known that alcohol, at its present price, cannot be used in this 
country on the large scale. I therefore endeavoured to find means of dispensing 
with its use, and I began by studying the properties of the soluble glass above 
mentioned, which contains only phosphoric acid, water, and magnesia. 
This glass dissolves slowly, but perfectly, in boiling water; but the solution, 
when again concentrated, and so far deprived of water, that its temperature in 
an open platinum capsule rises to nearly 600°, suddenly becomes turbid, from 
the separation of a powder, while crystals begin to form in the viscid mass, re- 
sembling those which form in honey. When cold, water dissolves these crystals 
instantly, and leaves undissolved only a heavy white powder, which is a peculiar 
phosphate of magnesia. I shall return to it presently. It is quite soluble in 
water. 
