PHOSPHATE OF MAGNESIA. 61 



ing as acid, is united to 2 eq. of magnesia, and 1 eq. of phosphoric acid, acting 

 as base, while a third eq. of phosphoric acid acts in another capacity, possibly as 

 a neutral body, like water of crystallization. 



In all these supposed formulae, we have anhydrous phosphoric acid acting in 

 an imusual capacity ; and it is evident that, whichever we adopt, the occurrence 

 of the salt is favourable to the doctrine, that the so-called anhydrous acids are 

 not really acids. It is true that, on the old view of phosphoi'ic acid and phos- 

 phates, there is nothing startling in a sesqui-phosphate ; hut, if we adopt this 

 view, we must cast aside all the knowledge recently acquired concerning the 

 phosphates, and which, to a great extent, is established by experiment. 



On the whole, the composition of this salt is so anomalous, that we must 

 conclude, either that the received views on the subject of the phosphates are 

 erroneous, or that there exists a fourth modification of phosphoric acid, distinct 

 from the three usually admitted. We might suppose this acid to he 3 P^ Og = 

 Pg O^g, and to be neutralised by 2 eq. base, yielding the formiila Pg 0^^, 2 M 0. 



It is worth while to remark, that, on the theory of compound acid radicals 

 and hydrogen acids, according to which the formula of a tribasic phosphate is 

 P, Og, M„^ that of a bibasic phosphate, P^ 0,., M.„ and that of a monobasic phos- 

 phate, P^ Og, M, this salt cannot be represented as a sesqui-metaphosphate, al- 

 though, as we have seen, it may be so on the old view of acids and salts. . The 

 latter makes it 3 Pg Og + 2 M _^ ; but the former would requu-e 1 eq. of oxygen 

 more to yield the formula of a sesqui-metaphosphate 3 P„ Og, ^g„, the salt contain- 



2 P 1 

 ing only „ - *^ I M <7o. In this point of view, the existence of the new salt may 



2 o j 



turn out to be a serious objection to the above named theory of salts and acids. 



The new phosphate is remarkable, among aU the salts of magnesia, for its 

 extreme insolubility in water and acids, Avhich is such, that it may be ranked be- 

 side sulphate of baryta. This insolubility has hitherto defeated all my efforts to 

 ascertain the precise nature of the acid it contains, and whether that acid be new 

 or not. 



As this salt is not only very insoluble, but easily washed, and less hygrome- 

 tric than any powder I have ever weighed, it would be well adapted for the deter- 

 mination of magnesia, and also of phosphoric acid, if we could, at pleasure, con- 

 vert these substances into this particular form of combination when mixed with 

 other bodies. The experiments I have made on this subject have convinced me, 

 that there are very great difficulties in the way of this application ; but, I trust, 

 they will not be found insuperable. It will, at all events, be easy to detennine 

 the amount of magnesia in bones, by converting the magnesia, as in the above 

 process, into this insoluble form. 



