Lime } } { lime } } Lime } phosphoric acid } { water} phosphoric acid } Lime } } + { water} } = (One molecule of insoluble } { (One molecule of soluble} phosphate) } { phosphate) } Lime } } { lime } Lime } phosphoric acid } { lime } phosphoric acid. Water} } + { water} (One molecule of reverted } { (One molecule of reverted phosphate) } { phosphate.) It may be mentioned, however, that reversion from this cause probably takes place to a very slight extent in practice.[230] Where reversion is due to the presence of iron and alumina in the raw material, the nature of the reaction is not well understood, and is consequently not so easily demonstrated as in the former case. Where iron is present in the form of pyrites, or ferrous silicate, it does not seem to cause reversion. It is only when it is present in the form of oxide--and in most raw phosphatic materials it is generally in this latter form[231]--that it causes reversion in the phosphate. _Value of reverted Phosphate._ The value of reverted phosphate is a subject which has given rise to much dispute among chemists. That it has a higher value than the ordinary insoluble phosphate is now admitted; but in this country, in the manure trade, this is not as yet recognised. At first it was thought that it was impossible to estimate its quantity by chemical analysis. This difficulty, however, has been overcome, and it is generally admitted that the ammonium citrate process furnishes an accurate means of determining its amount. Both on the Continent and in the United States reverted phosphate is recognised as possessing a monetary value in excess of that possessed by the ordinary insoluble phosphate. The result is, that raw phosphates containing iron and alumina to any appreciable extent are not used in this country, although they do find a limited application in America and on the Continent. _Composition of Superphosphates._