_Answer_. Such combination always gives out heat, whereas their separation absorbs heat. It has further been proved by actual experiment that the amount of heat liberated by the chemical union of the same quantity or number of atoms of two or more substances is always the same, and that when, by any cause, the atoms thus joined are separated, exactly the same amount of heat is absorbed.[88] [88] “The New Chemistry,” by J. P. Cooke, Jr. QUESTION 374. _In what proportions do the elementary substances combine with each other?_ _Answer_. It is a law of chemistry that each of the elementary substances combines with the others in certain definite proportions only. These proportions vary for the different elements, and have been determined with great accuracy by chemists. Thus, eight parts by weight of oxygen will combine with nitrogen and form atmospheric air, or the same proportion of oxygen will combine with hydrogen and form water, or with carbon and form carbonic acid, which is the deadly gas which accumulates at the bottom of wells. Now oxygen always combines with other substances in the proportion of eight parts by weight, or by _some simple multiple of_ eight, that is 8 × 2 = 16 parts, or 8 × 3 = 24 parts, etc. Each of the other elementary substances also has a certain fixed proportion in which it combines with others, and this proportion, which is usually given by weight, is represented by a number called its _chemical equivalent_. Thus 8 is the chemical equivalent of oxygen. Carbon combines with other elements in proportions of 6 and nitrogen in proportions of 14, so that 6 and 14 are the chemical equivalents of carbon and nitrogen. Now 8 parts by weight of oxygen can be made to combine with 14 parts of nitrogen, or 8 × 2 = 16 parts of oxygen will combine with 14 of nitrogen, but it is impossible to make, say 12 parts of oxygen combine with 14 parts of nitrogen. We can combine 14 × 2 = 28 parts of nitrogen with 8 parts of oxygen, but no chemical process can make say 10 or 20 parts of nitrogen combine with 8 parts of oxygen. If 20 parts of nitrogen are mixed with 8 parts of oxygen, then the latter will combine with 14 parts of the former, but 6 parts of nitrogen will be left, and chemical combination will then cease. The following table will give the chemical equivalents of the principal elements which enter into the process of combustion of the fuel used in locomotives: Chemical equivalent by weight. Oxygen 8 Nitrogen 14 Hydrogen 1 Carbon 6 Sulphur 16 QUESTION 375. _What effect do the proportions in which elements are combined have upon the substances which are produced by the combination?_