We remain thus far in the region of fact: why not rest there? It might as well be asked why we do not, like our poor relations of the woods and forests, rest content with the facts of the sensible world. In virtue of our mental idiosyncrasy, we demand _why_ bodies should combine in multiple proportions, and the outcome and answer of this question is the atomic theory. The definite weights of matter, above referred to, represent the weights of atoms, indivisible by any force which chemistry has hitherto brought to bear upon them. If matter were a _continuum_--if it were not rounded off, so to say, into these discrete atomic masses--the impassable breaches of continuity which the law of multiple proportions reveals, could not be accounted for. These atoms are what Maxwell finely calls "the foundation stones of the material universe," which, amid the wreck of composite matter, "remain unbroken and unworn." A group of atoms drawn and held together by what chemists term affinity is called a molecule. The ultimate parts of all compound bodies are molecules. A molecule of water, for example, consists of two atoms of hydrogen, which grasp and are grasped by one atom of oxygen. When water is converted into steam, the distances between the molecules are greatly augmented, but the molecules themselves continue intact. We must not, however, picture the constituent atoms of any molecule as held so rigidly together as to render intestine motion impossible. The interlocked atoms have still liberty of vibration, which may, under certain circumstances, become so intense as to shake the molecule asunder. Most molecules--probably all--are wrecked by intense heat, or in other words by intense vibratory motion; and many are wrecked by a very moderate heat of the proper quality. Indeed, a weak force, which bears a suitable relation to the constitution of the molecule, can, by timely savings and accumulations, accomplish what a strong force out of relation fails to achieve. We have here a glimpse of the world in which the physical philosopher for the most part resides. Science has been defined as "organized common sense;" by whom I have forgotten; but, unless we stretch unduly the definition of common sense, I think it is hardly applicable to this world of molecules. I should be inclined to ascribe the creation of that world to inspiration rather than to what is currently known as common sense. For the natural history sciences the definition may stand--hardly for the physical and mathematical sciences.