“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 no lime in the true sense, for it has no power to neutralize acid soils, except when the phosphorus is taken up by plants more rapidly than the calcium, which in such case might remain in the soil to act as a base to neutralize soil acids; but even then the effect of the small amount of calcium thus liberated from the phosphate would be very insignificant compared with a liberal application of ground limestone.”

“Well,” said Mr. Thornton, stretching himself, “orange phosphate is my favorite drink but I fear some of these phosphate you have just been giving me are too concentrated.  I ought to have the dose diluted; but I like the taste of it, and if you’ll write a book along this line, in this plain way just about as you have been giving it to me straight for almost twelve hours, I tell you I’ll read it over till I learn to understand it a heap better than I do now.”

CHAPTER XVIII

CLOSER TO MOTHER EARTH

The following day Percy collected soil samples to represent the common type of soil on the farm.  In the main the land was nearly level and very uniform, although here and there were small areas which varied from the main type, and in places the variation was marked.  Percy and his host devoted the entire day to an examination of the soils of the farm and the collection of the samples.