Intelligence and Miscella7ieous Articles. 321 



alkaline phosphates having formed with the earthy phosphates, double 

 salts which are insoluble in water but soluble in acids. 



Ammoniaco-magnesian phosphate is the only known double earthy 

 phosphate insoluble in water. But double salts which are absolutely 

 analogous may be formed with potash, soda, and even lithia, either 

 with magnesia or lime. 



When the phosphate of lime or magnesia is fused with an excess 

 of an alkaline carbonate, a double salt is formed. But by this pro- 

 cess it is not obtained pure, because the excess of the alkaline car- 

 bonate always effects partial decomposition. The higher the tempe- 

 rature is raised, the more complete is the fusion and the larger is the 

 quantity of earthy phosphates decomposed. With an atom of phos- 

 phate of magnesia mixed with an atom of potash or soda, complete 

 decomposition may be effected ; but with phosphate of lime this is 

 not the case ; the cause is, that the decomposition of the double salt 

 of the alkaline phosphate, and phosphate of lime, is effected with 

 difficulty by the alkaline carbonate. 



But when the earths are intimately mixed with a small proportion 

 of alkaline carbonate and heated, so that the mass neither fuses nor 

 sinks, all the carbonic acid is expelled from the alkali, and it is then 

 not possible to extract the alkali completely from the calcined mass 

 by water either cold or hot. 



It is diflficult to obtain this compound in a state of great purity. 

 This depends partly on the quantity of carbonate of potash employed. 

 If it be too small, the compound obtained is mixed with too much 

 earthy phosphate ; if too much be used, the decomposition is but 

 slight, and the compound contains a larger or smaller proportion of 

 earthy carbonate. The composition of the compound partly depends 

 also upon the degree of washing, which modifies it more or less con- 

 siderably. 



Make a very intimate mixture of an atom of earthy pyrophosphate 

 with an atom of an alkaline carbonate, and heat the mixture till it 

 ceases to lose weight. The calcined mass is then neither fused nor 

 sunk ; it is to be heated for some time in water, and washed with it 

 boiling. 



The compounds obtained yield by analysis greater or less approxi- 

 mations to a combination of two atoms of earth, one atom of alkali 

 and one atom of phosphoric acid ; so that the proportion of oxygen 

 of the bases approximates that of the phosphorus in the proportion 

 of 3 to 5. 



In the greater number of cases the washing requires much time, 

 and in some it seems interminable ; it appears, therefore, that the 

 alkali is obstinately retained ; but a large proportion, and frequently 

 the greater part, may be separated from the earthy phosphate by 

 prolonged washings. 



In these cases, the alkali removed by washing is replaced by an 

 equivalent of water ; so that a compound, washed for a long time 

 with hot water, is partly constituted of the original compound which 

 is not decomposed, and partly of a new combination of two atoms of 

 earth, one atom of water, and one atom of phosphoric acid, these 

 compounds existing either mixed or chemically combined. 



