nitrogen. Thus, when a piece of bright metallic sodium is exposed to the air, the surface becomes instantly tarnished and coated over with a film of oxide : when iron rusts, it in the same way is being acted upon by the oxygen of the air forming an oxide (or hydrated oxide) of iron ; in these cases the metals are said to become oxidised. If the metal be obtained in a sufficiently finely divided condition before being exposed to the air, or to pure oxygen, this process of oxidation may proceed so rapidly that the heat developed by the combination will cause the metal to burn. When the process of oxidation is accompanied by light and heat, the phenomenon is known as combustion^ the oxygen being spoken of as the supporter of combustion : bodies which burn in the air, therefore, are simply undergoing rapid combination with oxygen. It will obviously follow, that bodies which are capable of burning in the air will burn with greatly increased rapidity and brilliancy when their combustion is carried on in pure or undiluted oxygen. A glowing chip of wood, or a taper with a spark still upon the