But the simplest drop of water, in itself, and apart from its mechanical relations to other matter, is really a very complex and a very wonderful thing; not at all likely to be “self-caused.”  Water is made up, we know, of oxygen and hydrogen—­two elementary colourless, formless gases.  Now we can easily divide the one drop into two, and, without any great difficulty, the two into four, and (perhaps with the aid of a magnifying glass) the four into eight, and so on, as long as the minute particle still retains the nature of water.  In short, we speak of the smallest subdivision of which matter is capable without losing its own nature, as the molecule.  All matter may be regarded as consisting of a vast mass of these small molecules.

Now, we know that all known matter is capable of existing either in a solid, liquid, or gaseous form, its nature not being changed.  Water is very easily so dealt with.  Some substances, it is true, require very great pressure or very great cold, or both, to alter their form; but even carbonic acid, oxygen, and hydrogen, which under ordinary conditions are gases, can with proper appliances be made both liquid and solid.  Pure alcohol, has, I believe, never been made solid, but that is only because it is so difficult to get a sufficient degree of cold:  there is no doubt that it could be done.

It might be supposed that the molecules of which dead matter (whether solid, liquid, or vapourous) is composed, were equally motionless and structureless.  But it is not so:  every molecule in its own kind is endowed with marvellous properties.  In the first place, every molecule has a double capability of motion.  In the solid form the molecules are so packed together that, of course, the motion is excessively restricted; in the liquid it is a little easier; in the gaseous state the molecules are in a comparatively “open order.”  In most substances that are solid under ordinary conditions, by applying heat continuously we first liquefy and ultimately vapourize them.  In those substances which under ordinary conditions are gas (like carbonic acid, for instance), it is by applying cold, with perhaps great pressure as well, that we induce them to become liquid and solid; in fact, the process is just reversed.  As we can most easily follow the process of heating, I will describe that.  First, the solid (in most cases) gets larger and larger as it progresses to liquefaction, and when it gets to vapour, it suddenly expands enormously.  Take a rod of soft iron, and reduce it to freezing temperature:  let us suppose that in that condition it measures just a thousand inches long.  Then raise the temperature to 212 degrees (boiling point), and it will be found to measure 1,012 inches.  Why is that?  Obviously, because the molecules have got a little further apart.  If you heat it till the iron gets liquid, the liquid would also occupy still more space than the original solid rod; and if we had temperature high enough to make the melted iron go off into vapour, it would occupy an enormously increased space.  I cannot say what it would be for iron vapour; but if a given volume of water is converted into vapour, it will occupy about 1,700 times the space it did when liquid, though the weight would not be altered.