In order to clearly understand the nature of the reaction which takes place when sulphuric acid is added to a phosphatic material, it may be well to say a word or two on the composition of the different compounds of lime and phosphoric acid. _Phosphates of Lime._ In the various phosphatic manures used in agriculture there are four different kinds of phosphates. In the commonest form, popularly called bone-phosphate, which is the form in which lime and phosphoric acid are combined in bones, guano, and the ordinary mineral phosphates, the lime and phosphoric acid are combined in the form of what is known as tribasic phosphate of lime, or tricalcic phosphate--that is to say, for every equivalent of phosphoric acid there are three equivalents of lime. This may be represented as follows:-- Lime } Lime > Phosphoric acid. Lime } Or we may also say that for every 142 parts by weight of phosphoric acid there are 168 parts by weight of lime in this form of phosphate. This is the least soluble form of phosphoric acid,[226] and is the form generally referred to in commercial analyses as insoluble phosphate. When this phosphate is acted upon with sulphuric acid, a soluble phosphate is formed, as Liebig first showed, to which the name superphosphate has been given, and which is also known as monobasic phosphate of lime, or monocalcic phosphate. This compound may be represented as containing, instead of three equivalents of lime, only one, the other two equivalents being replaced by water. This compound may be represented as follows:-- Lime } Water > Phosphoric acid. Water } In it, for every 142 parts of phosphoric acid, there are only 56 parts of lime. It is soluble in water, and gives to the commercial article known as superphosphate of lime its value. Intermediate in composition between these two phosphates there is another known as precipitated phosphate of lime, or dicalcic phosphate (the same as reverted phosphate), which contains two equivalents of lime and one equivalent of water as follows:-- Lime } Lime > Phosphoric acid. Water } This compound contains, for every 142 parts of phosphoric acid, 112 parts of lime; and in solubility occupies an intermediate position. Lastly, there is a fourth compound of lime and phosphoric acid, which only occurs in one phosphatic manure--viz., phosphatic slag, in which indeed it was first discovered--which consists of four equivalents of lime to one of phosphoric acid, to which the name tetrabasic phosphate of lime or tetracalcic phosphate has been given. Its composition may be illustrated as follows:-- Lime } Lime > Phosphoric acid. Lime > Lime }