To these substances we may, in my opinion, give the title of Principles or Elements: at least, with regard to us, they are really such. Of this sort the principal are Earth, Water, Air, and Fire. For though there is ground to believe that these are not the primary component parts, or the most simple elements, of matter; yet, as we know by experience, that our senses cannot possibly discover the principles of which they are composed, it seems more reasonable to fix upon them, and consider them as simple homogeneous bodies, and the principles of the rest, than to fatigue our minds with vain conjectures about the parts or elements of which they may consist; seeing there is no criterion by which we can know whether we have hit upon the truth, or whether the notions we have formed are mere fancies. We shall therefore consider these four substances as the principles or elements of all the various compounds which nature presents to our inquiries: because, of all those we have as yet discovered, they are in fact the most simple; and because all our decompositions, all our experiments on other bodies, plainly prove that they are at last resolvable into these primary parts. These principles do not enter in the same proportion into all bodies: there are even some mixts in the composition of which this or that particular principle is not to be found. Thus Air and Water seem to be wholly excluded from the texture of Metals; at least all the experiments that have hitherto been made on them seem to confirm this opinion. The substances composed immediately of these _first_ Elements we shall call _secondary_ Principles; because in reality their several combinations with each other, the interchangeable coalitions that take place between them, constitute the different natures of all other bodies; which, as they result from the union both of primary and secondary principles, are properly entitled to the name of Compounds or Mixts. Before we enter upon the examination of Compound Substances, it is necessary to consider with some attention the most Simple ones, or our four first principles, in order to discover their chief properties. SECTION I. _Of_ AIR.