Every economic process leaves behind a waste product. A coal fire, whether in a poor man's cottage or an industrial boiler, leaves behind ash, soot, and carbon particles, which either escape into the air if they are finely divided, or remain as solids if they are not. In addition, waste gases — oxides of carbon and sulphur mostly — will be discharged into the air unless means exist to detain them. The smelting of metals leaves a large residue of dross and slag and much smoke and fume. A modern power station or cement works will shower the neighbourhood with dust unless carefully managed. Oil refineries and chemical works threaten to give off poisonous and evil-smelling gases. A coffee roaster can fill a shopping street with smoke and smells. Even a performance of Beethoven's Eroica Symphony will generate a little dust as the horsehair bows of the string players gradually wear out. Pure air is therefore like liberty — only to be had at the price of eternal vigilance. And in practice some degree of impurity or pollution can hardly be avoided. In a state of nature the air will not consist simply of oxygen, nitrogen, carbon dioxide and the rare inert gases. It will contain dust and particles blown by the wind as well as a varying amount of water vapour. Atmospheric pollution occurs when man, volcanoes, or fierce winds substantially change the normal composition of natural air. This statement does not amount to a clear-cut definition, but in practice gross pollution, like the dog, can usually be recognised even if it is hard to define.