Against carbon will be seen the number 12;
against oxygen, 16; and against hydrogen, 1. These numbers indicate that
if the smallest weight of hydrogen ever found in a chemical compound is
called 1 as a unit of comparison, the smallest weights of calcium,
carbon, and oxygen, similarly taking part in chemical reactions are 40,
12, and 16 respectively. Thus the symbol CaC_2, comes to convoy three
separate ideas: (_a_) that the substance referred to is a compound
of calcium and carbon only, and that it is therefore a carbide of
calcium; (_b_) that it is composed of one chemical part or atom of
calcium and two atoms of carbon; and (_c_) that it contains 40 parts
by weight of calcium combined with twice twelve, or 24, parts of carbon.
It follows from (_c_) that the weight of one chemical part, now
termed a molecule as the substance is a compound, of calcium carbide is
(40 + 2 x 12) = 64. By identical methods of calculation it will be found
that the weight of one molecule of water is 18; that of acetylene, 26;
and that of lime, 56. The general equation (1) given above, therefore,
states in chemical shorthand that 64 parts by weight of calcium carbide
react with 18 parts of water to give 26 parts by weight of acetylene and
56 parts of lime; and it is very important to observe that just as there
are the same number of chemical parts, viz.
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