Well, this smallest possible part of water (or any other form of Substance) is a Molecule. Remember the Molecule is the smallest part of that kind of Substance that can be produced by division and sub-division, without destroying the "kind" of the Substance. It is the smallest part of any kind of Substance that can exist by itself, and maintain its "kind." In order that you may grasp the minuteness of the Molecule, we may mention that Science claims that no molecule, even the largest, is of sufficient magnitude to be seen under even the strongest microscope. It has been calculated that if a drop of water as large as a pea were magnified to the size of the Earth, the molecules would then appear no larger than the original drop. The space between the molecules is believed to be considerably larger than the molecules themselves. The figures that are necessary to use in connection with molecular Substance are likely to stagger the imagination. Besides speaking of the molecules of inorganic Substance, it may be interesting to note that a spider's thread is so fine that a piece of it large enough to circle the earth would weigh only half a pound. And yet each thread is composed of six thousand filaments. And each of these minute filaments may be divided into tiny bits, and each bit will still be a Mass of Substance containing thousands of molecules and their constituent chemical atoms. There are living, microscopic creatures, so small that five millions of them might be crowded into a space the size of a pin head. And yet each of them have organs. And in these organs fluids circulate. Try to figure out the size of the molecules of the fluids circulating in these tiny organs, not to speak of the chemical atoms. When you handle a coin, an infinitesimal portion of it is worn off--can you figure the size of the molecules composing that part? When a rose throws off its perfume, it emanates tiny particles of itself--can you measure or weigh the molecules composing that odor? The human mind is compelled to realize its finiteness when it considers these things--but we have only just begun to consider the smallness of Things. _An "Atom"_ is the _chemical_ unit of Substance--that is, the smallest chemical part that can enter into combination. It has been considered indivisible--that is, incapable of further sub-division. That is, it has been so considered, until very recently, but the latest discoveries have exploded this idea, and have shown the Atom is composed of certain other Things, as we shall see a little later on. Still we may use the Atom as a very good unit of measurement, for it still represents the unit of _chemical_ Substance, just as the molecule is the unit of _physical_ Substance. In order that you may understand the difference between Molecules and Atoms--physical units, and chemical units, let us give you a few examples.