_Answer_. A change in the proportions in which the elements are combined usually alters the entire nature of the substance, so far at least as it affects our senses. For instance, oxygen unites chemically with nitrogen in different proportions, forming five distinct substances, each essentially different from the others, thus: 14 parts of Nitrogen with 8 of Oxygen forms Nitrous Oxide. 14 „ „ „ „ 16 „ „ „ Nitric Oxide. 14 „ „ „ „ 24 „ „ „ Hyponitrous Acid. 14 „ „ „ „ 32 „ „ „ Nitrous Acid. 14 „ „ „ „ 40 „ „ „ Nitric Acid. We here find the elements of the air we breathe, by a mere change in the _proportions_ in which they are united, forming distinct substances, which differ from each other as much as _laughing gas_ (nitrous oxide) does from that most destructive agent _nitric acid_, commonly called _aqua fortis_.[89] [89] Combustion of Coal and the Prevention of Smoke, by C. Wye Williams. QUESTION 376. _What occurs when a fresh supply of bituminous coal is thrown on a bright fire in the fire-box of a locomotive?_ _Answer_. The fresh coal is first heated by the fire, and if a sufficient quantity is thrown in to prevent the immediate formation of flame,[90] a volume of gas or vapor, usually of a dark yellow or brown color, is given off. The quantity evolved will be greatest when the coal is very small. This gas or vapor is commonly called smoke, but it does not deposit soot and in reality is not true smoke. If a sheet of white paper be held over the vapor as it escapes from the coal and there is no flame, the sheet will become slowly coated with a sticky matter of brown color difficult to remove, and having a strong tarry or sulphurous smell; whereas if a sheet of paper is held over smoke it will quickly be covered with black soot. The color and smell left on the paper in the first case are due to the tarry matter, sulphur, and other ingredients in the gas. Deprived of the coloring matters, the vapor is a chemical mixture of 2 parts of hydrogen and 6 parts of carbon, and is called carburetted hydrogen, and is nearly the same as the colorless gas by which our houses are lighted.[91] A similar gas is generated at the wick of a burning candle or lamp and is consumed in the flame. Before the gas is expelled from the fresh coal the latter must be heated to a temperature of about 1,200 degrees, so that if 100 pounds at a temperature of 50 degrees is put on the fire 23,000 units of heat will be absorbed to heat the coal.[92] Nor is this all, as has been explained in answer to Question 38, when any substance is vaporized a certain amount of heat apparently disappears, which has been called the _heat of evaporation_ or _of gasification_. Average bituminous coal contains about 80 per cent. of carbon, 5 per cent. of hydrogen and 15 per cent. of other substances usually regarded as impurities.