L., is meant tricalcium phosphate, a compound which contains exactly
twenty per cent. of phosphorus. Thus, you can always divide the
guaranteed percentage of 'bone phosphate of lime' by five, and the
result will be the per cent. of phosphorus.

"As stated in your Century dictionary, true phosphoric acid forms
three distinct classes of salts, because either one, two, or all of
the three hydrogen atoms may be replaced by a metallic element.
Thus, we have phosphoric acid itself containing the three hydrogen
atoms, one phosphorus atom, and four oxygen atoms. This might be
called trihydrogen phosphate (H3PO4). Now if one of the hydrogen
atoms is replaced by one potassium atom, we have potassium
dihydrogen phosphate (KH2PO4); with two potassium atoms and one
hydrogen, we have dipotassium hydrogen phosphate (K2HPO4); and if
all hydrogen is replaced by potassium the compound is tripotassium
phosphate (K3PO4). To make similar salts with two-handed metallic
elements, like calcium or magnesium, we need to start with two
molecules of phosphoric acid H6(PO4)2; because each atom of calcium
will replace two hydrogen atoms. Thus we have mono calcium
phosphate, CaH4(PO4)2, dicalcium phosphate, Ca2H2(PO4)2, and
tricalcium phosphate, Ca3(PO4)2. It goes without saying that
monocalcium phosphate contains four atoms of hydrogen and that
dicalcium phosphate contains two hydrogen atoms. By knowing the
atomic weights (40 for calcium, 31 for phosphorus, and 16 for
oxygen), it is easy to compute that the molecule of tricalcium
phosphate weighs 310 of which 62 is phosphorus. This is exactly
one-fifth, or twenty per cent. This compound you will remember is
sometimes called 'bone phosphate of lime'. It is also called simply
'bone phosphate'; because it is the phosphorus compound contained in
bones. It is sometimes called lime phosphate, although it contains