8 SCIENCE AND SOIL 



oxid (CaO). The atomic weight of calcium is 40. The subscript 

 figures used in a chemical formula always refer to the preceding 

 element, or to the inclosed group of elements if parentheses 

 are used. In the calcium phosphate, Ca 3 (PO 4 ) 2 , the subscript 3 

 shows that three atoms of calcium are contained in the molecule. 

 The parentheses are used to inclose a group of atoms (one atom 

 of phosphorus and four atoms of oxygen) which occurs in many 

 other compounds, as in H 3 PO 4 (phosphoric acid), FePO 4 (iron 

 phosphate) , etc. (Fe is from ferrum, the Latin word for iron, and 

 I is the symbol used for the element iodin.) The subscript 2 follow- 

 ing the parenthesis in Ca 3 (PO 4 ) 2 means that the molecule contains 

 the PO 4 group twice, and for this reason Ca 3 (PO 4 ) 2 is a better for- 

 mula than Ca 3 P 2 O 8 , which may also be correctly used. A mole- 

 cule of tricalcium phosphate contains three atoms of calcium 

 (3 x 40 = 120), two atoms of phosphorus (2 x 31 = 62), and eight 

 atoms of oxygen (2 x 4 x 16 = 128), and the molecular weight is 

 310 (120 + 62 + 128), of which the phosphorus represents only 62. 

 Thus, tricalcium phosphate contains -/^$, or 20 per cent, of the 

 element phosphorus. In other words, one fifth of pure tricalcium 

 phosphate is phosphorus, while the remaining four fifths consist 

 of calcium and oxygen in nearly equal parts. 



The law of constant proportions. This law is that in every chemi- 

 cal combination the constituents occur in definite and constant 

 proportions by weight. The percentage of phosphorus in pure 

 tricalcium phosphate is absolutely constant. It matters not 

 whether the compound is made in the United States, in Germany, 

 or in Japan, nor whether it is obtained from bones or from phos- 

 phate rock, the percentage of phosphorus it contains is always 

 the same, if the compound is pure. This percentage is exactly 20, 

 according to the most accurate accepted determinations. The 

 atomic weights are determined by several different methods, but, 

 even with the finest and most accurate balances and other instru- 

 ments and means, absolute exactness may not be achieved, be- 

 cause of the human error. No man can measure a mile with 

 absolute exactness, because two different measurements made by 

 one man may vary by an inch, a tenth of an inch, or, at least, by 

 a hundredth or a thousandth of an inch. 



According to the chemical law, the proportion of the different 



