THE ancient philosophers divided all things into four elements--earth, water, air, and fire. Of these, or a mixture of them, they thought that all visible things were made up. Nowadays we recognise about seventy different substances, which we call elements, and we know that what the ancients called earth is a mixture in various proportions of about a dozen of our elements. When we use the word element, we mean a kind of substance which cannot be obtained by mixing other substances together, and cannot be split up into two or more substances possessing different properties. We know that water can be split up into two substances, and that air is a mixture of at least two other substances. As regards fire, it is now generally thought that it is composed of hot bodies having the consistency of air. Such bodies we now call gases. One of the most familiar examples of gas is that which is used for lighting a room or a street. But the gas is only one of a great variety of similar substances. Instead of four elements, we now speak of three states of matter. These three states are the solid state, the liquid state, and the gaseous state. When a substance has a certain amount of hardness, when it does not flow, and does not assume a level surface if left to itself, we call it a solid. If a substance flows, and fills up a vessel into which we pour it, and yet remains visible and tangible, we call it a liquid. Lastly, if the substance fills up completely any space into which we place it, and expands as far as it can, we call it a gas. Nearly every liquid and most solids may be converted into gases by heat, and this circumstance accounts for the great variety of gases which are known to exist. A great difference between a solid and a liquid is that a solid left to itself only presses down upon its support, whereas a liquid also presses sideways and upwards. In this respect fine sand partakes somewhat of the nature of both solid and liquid. If the stalk of a dandelion, or some other hollow stalk, is planted in a box of sand, it can be crushed by packing the sand very tightly. This shows that the sand is capable of exerting a pressure sideways. But the coarser the grains of sand, the less readily does this sideways pressure follow upon the pressure on the surface. On the other hand, if the sand is extremely fine it readily exerts pressure in all directions. If we could produce sand about a thousand times more finely grained than the finest sand known, we should obtain a substance which would instantly transmit the slightest pressure on the surface through the whole of the substance. We should have, in fact, a liquid.