One of-these speculations was that things which we see around us were built up out of elements, just as a word is built up out of letters. Indeed, the word "elemens", which is the Latin word for element, is probably derived from the letters l, m, and n, and involves that idea. The ancient Greeks surmised that there were four such elements—earth, water, air, and fire. But as it was obvious that some things, for instance gold and silver, did not contain either water or air, the word element was often used to signify, not the constituent of a thing, but rather a property of a thing; and it might have been said that gold partook of the properties of earth and water, because, like earth, it is not altered by being heated, and yet it can take a fluid form like water if heated hot enough. Hence the old word "element" had a double meaning; it was sometimes used in the sense of " constituent," and sometimes more in the sense of " property".

If a child is given a mechanical toy, his wish to see how it works generally leads to his taking it to bits. This is unfortunately only too easy; but it is seldom that he succeeds in putting it together again. Now, if we inquire what a piece of wood or stone is made of, we can, after a fashion, take them to bits ; we may pull the wood into fibres, or we may crush the stone, and pick out the pieces that appear to differ from each other in colour, if they are large enough. But the fibres have much the same appearance as the piece of wood, and the fragments of stone, though somewhat different from each other, are still pieces of stone. The question is still to be answered, of what do wood and stone consist ? It is evident that some plan must be tried by which the wood and stone will be unbuilt, as it were, and by which they will yield their constituents.

It had long been noticed that many things are greatly changed when heated. A piece of wood takes fire and burns ; some kinds of stone melt; some metals, such as lead and iron, change into earthy-coloured powders. Surely these changes ought to lead to some knowledge of the nature of wood, stone, and metals. It was long, however, before it was recognised that the presence or absence of air made a difference in the result of heating substances. When attention was drawn to this difference, a new suggestion was adopted. It was, that things, besides consisting of or sharing the properties of earth, water, air, and fire, also consist of, or at least are like, salt, sulphur, and mercury. Salt dissolves when put into water ; so do many other things. These things must either contain a kind of salt to account for this property; or they must at least share the property of salt, in so far as they dissolve. Similarly, other things, especially metals, must either contain or share the property of mercury, seeing that they shine with the same kind of lustre; and many things resemble sulphur in so far as they burn and produce a smell in burning. And it was often imagined that when things burn, the sulphur which they contain flies away and disappears, just as ordinary sulphur, when set on fire, burns away completely, leaving nothing behind. About the middle of the seventeenth century, Johann Joachim Becher, a German alchemist, altered somewhat the conception that substances contain, or are like, salt, sulphur, and mercury; he imagined all things existing on the surface of the globe to contain three earths, namely the mercurial, the glassy, and the fatty, the last implying the property of being able to burn. And in. the early years of the eighteenth century, Becher's pupil, George Ernest Stahl, who was Professor of Medicine in Jena, and later in Halle, two small German towns, made an important addition to the ancient theories, namely, that it was possible to restore the " sulphur," or the " fatty earth," as Becher called it, to things which had been deprived of it by burning, by heating them with other substances rich in that constituent: