This equation not only means that calcium carbide and water combine to yield acetylene and lime, it also means that one chemical part of carbide reacts with one chemical part of water to produce one chemical part of acetylene and one of lime. But these four chemical parts, or molecules, which are all equal chemically, are not equal in weight; although, according to a common law of chemistry, they each bear a fixed proportion to one another. Reference to the table of "Atomic Weights" contained in any text-book of chemistry will show that while the symbol Ca is used, for convenience, as a contraction or sign for the element calcium simply, it bears a more important quantitative significance, for to it will be found assigned the number 40. 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., 2, on each side, so there are the same number of parts by weight, for 64 + 18 = 56 + 26 = 82. Put into other words equation (1) shows that if 64 grammes, lb., or cwts. of calcium carbide are treated with 18 grammes, lb., or cwts. of water, the whole mass will be converted into acetylene and lime, and the residue will not contain any unaltered calcium carbide or any water; whence it may be inferred, as is the fact, that if the weights of carbide and water originally taken do not stand to one another in the ratio 64 : 18, both substances cannot be entirely decomposed, but a certain quantity of the one which was in excess will be left unattacked, and that quantity will be in exact accordance with the amount of the said excess--indifferently whether the superabundant substance be carbide or water.