The smallest part of any substance capable of independent existence is called a molecule, and according to Avogadro’s law already quoted the number of molecules in a given volume of any gas, under like conditions, is the same, and is independent of the composition of the gas. It requires a little thought to realise how tiny are these particles and how many there are crowded together in any small portion of a gas. In gases also they are much further apart than in a liquid or solid. As to their size no better idea can be conveyed than in the words of Lord Kelvin in a lecture at the Royal Institution in 1883. He says: “To form some conception of the degree of coarse-grained-ness indicated imagine a globe of water or glass, as large as a football (or say a globe 16 centimetres diameter) to be magnified up to the size of the earth, each constituent molecule being magnified in the same proportion. The magnified structure would be more coarse-grained than a heap of small shot, but probably less coarse-grained than a heap of footballs.”