48 PROFESSOR GREGORY ON THE EXTRACTION OF 



show 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 Liebig, founded on 

 the fact, that the whole lime may be removed from the superphosphate of lime, 

 bv the addition of sulphiiric 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 Avhole 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. 



Liebig acts on the concentrated acid solution, lirought to the consistence of 

 s}Tup, by alcohol, which dissolves the phosphoric acid, leaving imdissolved 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 

 oxpellhig 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 

 Liebig 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 oj)en 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. AVhen cold, water dissolves these crj'stals 

 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 insoluble in 

 water. 



