All the elements, with the single exception of fluorine, combine with oxygen to form oxides. In this act of combination, which is termed oxidation, heat is always, and light is frequently, given off. When bodies unite with oxygen, evolving light and heat, they are said to burn, or undergo combustion. All bodies which burn in the air burn with increased brilliancy in oxygen gas; and many substances, such as iron, which do not readily burn in the air, may be made to do so in oxygen. Oxygen is a colorless invisible gas, possessing neither taste nor smell. Hydrogen is a colorless invisible gas, possessing neither taste nor smell. It is the lightest gas known, being 14.47 times lighter than air. It combines with oxygen to form water. Nitrogen is a colorless, tasteless, inodorous gas, slightly lighter than air. It does not combine readily with bodies, and it is a very inert substance, neither supporting combustion or animal life, nor burning itself. It has, however, no poisonous qualities, and animals plunged into a jar of this gas die simply of suffocation from want of oxygen. Nitrogen exists in a free state in the air, of which it constitutes four-fifths by bulk. It occurs combined in the bodies of plants and animals, and in various chemical compounds, such as nitre, whence the gas derives its name. Carbon is a solid element; it is not known in the free state, either as a liquid or as a gas. Carbon is remarkable as existing in three distinct forms, which in outward appearance or physical properties have nothing in common, whilst their chemical relations are identical. These three allotropic forms of carbon are (1) diamond, (2) graphite or plumbago, (3) charcoal. These substances differ in hardness, color, specific gravity, etc., but they each yield on combustion in the air or oxygen the same weight of the same substance, carbonic acid or carbon dioxide. Carbon is the element which is especially characteristic of animal and vegetable life, as every organized structure, from the simplest to the most complicated, contains carbon. If carbon were not present on the earth, no single vegetable or animal body such as we know could exist. In addition to the carbon which is found free in these three forms, and contained combined with hydrogen and oxygen in the bodies of plants and animals, it exists combined with oxygen as free carbon dioxide in the air, and with calcium and oxygen as calcium carbonate in limestone, chalk, marble, corals, shells, etc. Plants are able when exposed to sunlight to decompose the carbon dioxide in the air, liberating the oxygen, and taking the carbon for the formation of their vegetable structure, whilst all animals, living directly or indirectly upon vegetables, absorb oxygen, and evolve carbon dioxide. Thus the sun's rays, through the medium of plants, effect deoxidation, or reduction, whilst animals act as oxidizing agents with respect to carbon.