In the first place we know surely that there are natural units of matter. This was the great discovery made by Dalton in the beginning of the nineteenth century. When he found that each of the known elements, such as copper or oxygen or carbon, consisted ultimately of atoms, all the atoms of any one element being alike, he laid the foundation on which the huge structure of modern chemistry has been raised. The chemist takes one or more atoms of one element, one or more of another, and may be of a third or fourth, and he puts them together into a compound which we call a molecule. The molecule for example of ordinary salt contains always one atom of chlorine and one of sodium. Chlorine and sodium are elements, salt is a compound. Six atoms of carbon and six of hydrogen put together in a certain way make benzene. In the same way every substance that we meet is capable of analysis, showing ultimately the molecules as made up, according to a definite plan, of so many atoms of the various elements. In analytical chemistry molecules are dissected in order to discover the mode of their building; in synthetic chemistry the atoms are put together to make a molecule which is already known to have, or even may be anticipated to have, certain properties. This is the work of the chemist. Sometimes enormous forces are concerned in this pulling apart and putting together, witness the terrific power of modern explosives. But the same kind of handling by the chemist may be devoted to the delicate construction of a molecule which gives a certain colour to the dyer's vat and so pleases the eye that the great cloth industries feel the consequence, and nations themselves are affected by the flow of trade. After all, since the processes of the physical world operate ultimately through the power and properties of molecules, it is not surprising that the chemist's work in these and numberless other ways has such tremendous influence in the world. Here then by the recognition of the units of matter which Nature has chosen for herself it has been possible to do great things. It should be observed that the atom, in spite of its name, is not something which is incapable of all further division; it is only incapable of retaining its properties on division. When an atom of radium breaks down in the unique operation during which its singular properties are manifested, it dies as radium and becomes two atoms, one of helium, the other of a different and rare substance. It will interest you to know that the airships of the future are expected to be filled with this non-inflammable helium.