One of the most commonly occurring and most noticeable changes in the properties of matter is that which proceeds when a piece of wood, or a candle, or a quantity of oil burns. The solid wood, or candle, or the liquid oil slowly disappears, and this disappearance is attended with the visible formation of flame. Even the heavy fixed metals, tin or lead, may be caused to burn; light is produced, a part of the metal seems to disappear, and a white (or reddish) solid, very different from the original metal, remains. The process of burning presents all those peculiarities which are fitted to strike an observer of the changes of Nature; that is, which are fitted to strike a chemist--for chemistry has always been recognized as having for its object to explain the changes which matter undergoes. The chemists of the seventeenth and eighteenth centuries were chiefly occupied in trying to explain this process of burning or combustion. Van Helmont (1577-1644), who was a physician and chemist of Brussels, clearly distinguished between common air and other "airs" or gases produced in different ways. Robert Hooke (1635-1703), one of the original Fellows of the Royal Society, in the "Micographia, or Philosophical Description of Minute Bodies," published in 1665, concluded from the results of numerous experiments that there exists in common air a peculiar kind of gas, similar to, or perhaps identical with the gas or air which is got by heating saltpetre; and he further supposed that when a solid burns, it is dissolved by (or we should now say, it is converted into a gas by combining with) this peculiar constituent of the air. John Mayow (1645-1679), a physician of Oxford, experimented on the basis of facts established by Hooke. He showed that when a substance, _e.g._ a candle, burns in air, the volume of air is thereby lessened. To that portion of the air which had _dissolved_ the burned substance he gave the name of _nitre-air_, and he argued that this air exists in condensed form in nitre, because sulphur burns when heated with nitre in absence of common air. Mayow added the most important fact--a fact which was forgotten by many later experimenters--that the solid substance obtained by burning a metal in air weighs more than the metal itself did before burning. He explained this increase in weight by saying that the burning metal absorbs particles of "nitre-air" from the atmosphere. Thus Hooke and Mayow had really established the fact that common air consists of more than one definite kind of matter--in other words, that common air is not an element; but until recent times the term "element" or "elementary principle" was used without any definite meaning. When we say that the ancients and the alchemists recognized four elements--earth, air, fire, and water--we do not attach to the word "element" the same definite meaning as when we now say, "Iron is an element."