BEFORE the time of Lavoisier ideas concerning the nature of matter were mere speculations. Following the introduction, at the close of the eighteenth century, of the conception of the indestructibility of matter, and more especially with the introduction of the atomic theory a decade or so later, the idea of some sixty or seventy absolutely different kinds of matter received general acceptance. The unity of these different elements was, indeed, held by some, but as a pure speculation, while the evidence was all against it. It remained for the Periodic Law to show that there is a connection between these different elements. It is true, we are as far as ever from any knowledge of what that connection is, or from any knowledge of the nature of that primal substance out of which all matter is shaped, unless, indeed, the recent work of J. J. Thomson and others on the electric condition of gases is pointing us thitherward.
The early attempts to classify substances from a chemical, or rather alchemistical standpoint, were wholly superficial. Pliny, for example, describes two forms of lead, plumbum nigrum and plumbum candidum. The former term was used for lead proper, the latter for tin, though these two metals have little resemblance, except in their low melting points. Sulfuric acid was classed with the oils, as oil of vitriol, and the name has popularly and technically remained to the present, although the only resemblance of sulfuric acid to an oil is in its appearance. The chlorids of antimony and of tin were known respectively as butter of antimony and butter of tin, from the fact that they are semisolid substances, of much the same consistency as butter from milk. Even to-day we speak familiarly of milk of lime and milk of sulfur, though but for the fact that they are whitish liquids, they have nothing in common with the product from the cow. Perhaps to us one of the most remarkable instances of classification was the association of the black oxid of manganese with the white oxid of magnesium, commonly known as calcined magnesia. The only property common to these two, magnesia nigra and magnesia alba, as they were early called, is that both are fine powders. In the seventeenth and eighteenth centuries the discovery of the different gases began, and to the workers of that day all were but different kinds of air. Thus we find 'inflammable air* as the name for hydrogen, 'fixed air* for carbonic acid gas, and 'dephlogisticated marine acid air' for chlorin. That no better principle of