[90] In order that these laws may hold good, it is, of course, necessary that the substances are weighed under precisely similar conditions. To state these laws in a more absolute form, we can replace the term “weight” by “mass,” or in preference, “inertia”; for the inertias of bodies are proportional to their weights, providing that they are weighed under precisely similar conditions. For a discussion of the exact significance of these terms “mass” and “inertia,” the reader is referred to the present writer’s _Matter, Spirit and the Cosmos_ (Rider, 1910), Chapter I., “On the Doctrine of the Indestructibility of Matter.”
As examples of these laws we may take the few following simple facts:--
1. Pure water is found always to consist of oxygen and hydrogen combined in the ratio of 1·008 parts by weight of the latter to 8 parts by weight of the former; and pure sulphur-dioxide, to take another example, is found always to consist of sulphur and oxygen combined in the ratio of 8·02 parts by weight of sulphur to 8 parts by weight of oxygen. (The Law of Constant Proportion.)
2. Another compound is known consisting only of oxygen and hydrogen, which, however, differs entirely in its properties from water. It is found always to consist of oxygen and hydrogen combined in the ratio of 1·008 parts by weight of the latter to 16 parts by weight of the former, _i.e._, in it a definite weight of hydrogen is combined with an amount of oxygen _exactly twice_ that which is combined with the same weight of hydrogen in water. No definite compound has been discovered with a constitution intermediate between these two. Other compounds consisting only of sulphur and oxygen are also known. One of these (viz., sulphur-trioxide, or sulphuric anhydride) is found always to consist of sulphur and oxygen combined in the ratio of 5·35 parts by weight of sulphur to 8 parts by weight of oxygen. We see, therefore, that the weights of sulphur combined with a definite weight of oxygen in the two compounds called respectively “sulphur-dioxide” and “sulphur-trioxide,” are in the proportion of 8·02 to 5·35, _i.e._, 3 : 2. Similar simple ratios are obtained in the case of all the other compounds. (The Law of Multiple Proportions.)