§ =73.= Robert Boyle (1626-1691) had defined an element as a substance which could not be decomposed, but which could enter into combination with other elements giving compounds capable of decomposition into these original elements. Hence, the metals were classed among the elements, since they had defied all attempts to decompose them. Now, it must be noted that this definition is of a negative character, and, although it is convenient to term “elements” all substances which have so far defied decomposition, it is a matter of impossibility to decide what substances are true elements with absolute certainty; and the possibility, however faint, that gold and other metals are of a compound nature, and hence the possibility of preparing gold from the “base” metals or other substances, must always remain. This uncertainty regarding the elements appears to have generally been recognised by the new school of chemists, but this having been so, it is the more surprising that their criticism of alchemistic art was not less severe. The Stoichiometric Laws. § =74.= With the study of the relative weights in which substances combine, certain generalisations or “natural laws” of supreme importance were discovered. These stoichiometric laws, as they are called, are as follows:-- 1. “The Law of Constant Proportion”--_The same chemical compound always contains the same elements, and there is a constant ratio between the weights of the constituent elements present._ 2. “The Law of Multiple Proportions”--_If two substances combine chemically in more than one proportion, the weights of the one which combine with a given weight of the other, stand in a simple rational ratio to one another._ 3. “The Law of Combining Weights”--_Substances combine either in the ratio of their combining numbers, or in simple rational multiples or submultiples of these numbers._ (The weights of different substances which combine with a given weight of some particular substance, which is taken as the unit, are called the combining numbers of such substances with reference to this unit. The usual unit now chosen is 8 grammes of Oxygen.)[90] [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:--