We begin with _carbon_, which forms the chief constituent of all our combustibles. Some specimens of graphite, plumbago, or “blacklead” consist of almost pure carbon (98 per cent.), and some varieties of wood charcoal exceptionally contain 96 per cent.; but in ordinary charcoal the percentage is much less. Coal, the most familiar of our solid fuels, varies greatly in composition, carbon being the predominating constituent, in amount from 57 to 93 per cent. Coke, another fuel much used in metallurgical operations, is made by heating coal without access of air, when a large quantity of gaseous substances is expelled. Coke burns with an intense and steady heat without emitting any visible smoke, but it does not ignite as readily as coal. Carbon forms two different compounds with oxygen: both are invisible gases, but they differ in the proportions of the constituents, and present different properties. When carbon (coal, coke, or charcoal) is completely burnt, that is, with an abundant supply of air, the product is _carbonic acid_ gas, in which 3 parts of carbon are combined with 8 of oxygen: when, on the other hand, the carbon is burnt with a sufficiently restricted access of air, the result is _carbonic oxide_ gas, in which 3 parts of carbon are united with only 4 of oxygen. The reader will here observe that the former contains just twice as much oxygen as the latter for the same quantity of carbon. This fact and numberless others like it are expressed or summed up by another _law_ of chemical combination which states that when two elements combine in several different proportions these are invariably such that the ratios in the several compounds will be found to have exact and simple numerical relations; that is, such as may, when reduced to their lowest terms, be expressed by the simple integers 1, 2, 3, etc., as 1 : 2, 3 : 2; ... 8 : 9, etc. It comes to the same thing if we compare together the weights A and A´ which are united in each compound with any one identical weight of B, giving of course the ratio A : B ÷ A´ : B. For instance, in the case just given, of carbon and oxygen, 3 : 4 ÷ 3 : 8 = 2 : 1. This, which is simply stating the facts, is called the _law of multiple proportions_. On a later page will be found another illustration (see Index, _Nitrogen and Oxygen Compounds_), and its expression in terms of the _atomic theory_, which goes behind the facts (so to speak), but is extremely useful by comprehending many other groups of facts in chemistry and in other sciences. Carbonic acid gas is of course incombustible, but carbonic oxide gas burns by uniting with the additional proportion of oxygen and becoming carbonic acid. On the other hand, carbonic acid gas passing over red-hot coals takes up from them the additional proportion of carbon, and is, we may say, _unburnt_ into carbonic oxide. When we see a pale blue flame flickering over the bright embers in a fire grate, it is