It was, of course, well known that mere _mixtures_ were entirely different things from chemical _compounds_. We can mix sand and sugar together, but they remain sand and sugar, and can be separated again, having undergone no change. Or we can mix together two liquids or two gases, and they also can again be separated by suitable means. But when two substances chemically combine one with another, then we have some third thing which is entirely different from the original two, and which possesses properties dissimilar from either. Now, what has happened when substances thus combine? What are the laws of such combinations? And what are the ultimate constituents of matter, which render these combinations possible? Dalton was the first to undertake an explanation of these phenomena, backed up by experimental evidence. The historic importance of this cannot be overestimated. As Dr. Raphael Meldola says, in his “Chemistry”:-- “The doctrine of equivalence, even in its most elastic form, is still nothing more than a quantitative expression of the facts of chemical composition. Of course, there must be some underlying principle--some explanation of this simplicity of multiplicity. Such explanation was first definitely formulated in 1807-08 by John Dalton, who not only discovered the law of Multiple Proportions, but suggested a theory, the introduction of which marks one of the greatest epochs in the history of Chemistry. The reason why combination takes place in definite proportions by weight, and why, when the same element has more than one equivalent the principle of integral multiples is maintained is, according to Dalton’s explanation, because the combination is between the ultimate particles of which elementary matter is composed. This is the notion of the discontinuity or discreteness of matter. The “particles” of which matter is composed--whatever its state of aggregation--are, from Dalton’s point of view, ultimate in the sense of being indivisible. For this reason he called them _atoms_.” THE ATOMIC THEORY Here, then, we have at last the Atomic Theory--the theory, that is, that all matter, in all its stages, is built-up of extremely small particles which are so small, indeed, that they can no longer be sub-divided. They are the ultimate of matter--the “building stones of the Universe”--of which everything, animate and inanimate, is composed. These atoms were held to be spherical in shape, of a certain definite weight and figure, according to the element or substance in question. Thus: “every particle of water is like every other particle of water, every particle of hydrogen is like every other particle of hydrogen, etc.” These ultimate particles--atoms--were held to be indestructible. These atoms all had their own particular _weights_, which might be denoted by number. Hence “atomic weight.” These atoms, then, combine, forming molecules, or compounds of atoms; and molecules make up matter as we see and know it.