IT being agreed that heat is the agent which does work in an engine, and that steam, air, and vapor are but the instruments for transmitting the motion of heat to the machinery, our object will be to store up in an elastic working substance the heat derived from fuel, and to guard against loss as far as possible. As a general rule chemical combination is accompanied by the evolution or production of heat, and chemical decomposition by the disappearance of heat equal in amount to that produced during the previous combination of the elements which are undergoing separation. Combustion, or burning, is the name given to rapid chemical combination attended with the evolution of intense heat. It is necessary to bear these facts in mind in estimating the heating effect of fuel. Thus, where hydrogen and oxygen exist in coal in the proportion necessary for forming water (viz: one of hydrogen to eight of oxygen by weight,) it is usual to assume that they do not influence the heat of combustion. The hydrogen is taken to have been already burnt in oxygen. In coal there may be 5 per cent. of hydrogen, and 4 per cent. of oxygen: this would leave 41⁄2 per cent. of hydrogen available for heating purposes. There appear to be exceptions to the above rule, and Dr. Percy gives the result of an experiment where-two coals closely agreeing in ultimate composition have been found to differ by 5 per cent. in heating power. The composition of various kinds of coal is given by Dr. Percy, in his work on fuel, and it is well known that the differences in the constituent parts of coal are very great, and give rise to qualities of various kinds which influence the selection to be made for heating purposes. The heat given out in the burning of hydroand carbon is estimated as follows: gen I pound of hydrogen consumes about 36 I pound of carbon burnt to carbonic oxide, UNITS OF HEAT. 62,032. 4,452. 14,545. According to Dr. Percy the heating power of a substance is the number of units of heat produced by the combustion of a unit of weight of the substance; and if the unit of heat be defined according to the Centigrade scale Heating power of hydrogen is 34,462. Heating power of carbon is 2,473, when burnt to carbonic oxide. Heating power of carbon is 8,080, when burnt to carbonic acid. Also I pound of hydrogen evaporates 64.2 pounds of water at 212° Fah. Also I pound of carbon (burnt to carbonic oxide) evaporates 4.6 pounds of water at 212° Fah. Also I pound of carbon (burnt to carbonic acid) evaporates 15 pounds of water at 212° Fah. It does not appear that the absolute heat of combustion can be increased, but it is easy to pile up the waves of heat in an enclosed space, and thereby to increase wonderfully the apparent power of the combustion. The furnace may be looked upon as a large chemical apparatus in which coal and air are to be mixed together in the proportion best adapted for burning the fuel without waste. In performing this operation, an engineer falls very far behind a scientific chemist when operating on a small scale in his laboratory. Thus a chemist in burning one pound of ordinary coal in a carefully protected chamber, would cause the heat from the fuel to evaporate (say) 14 pounds of water, whereas the evaporation per pound of coal in a steam boiler seldom exceeds 10 pounds, or 101⁄2 pounds, of water, a common performance being the evaporation of from 6 pounds to 8 pounds of water. Looking at the question as one of admixture of fuel and air, the rough numbers usually quoted on the authority of Rankine are the following: For the actual burning of ordinary coal in a furnace, 12 pounds of air are required in order to combine with the constituents of each I pound of coal. But the gaseous products of combustion must be largely diluted, otherwise the air would not get at the fuel, and for this dilution as much air again is required, making a supply of 24 pounds of air to each 1 pound of fuel. Thirteen cubic feet of air, at 60° Fah. and under a pressure of 30 inches of mercury, weigh about 1 pound. Therefore 312 cubic feet of air are required for each 1 pound of fuel, which comes to nearly 700,000 cubic feet of air for the effective burning of one ton of coal. That gas and hydrocarbon vapor proceeding from coal require a good supply of air for burning was frequently shown by Faraday in a lecture experiment, and his illustration goes to the substance of the whole matter. The device was to soak a little cotton-wool in any hydrocarbon liquid, and set it on fire in a jar of oxygen gas. In such a case the hydrogen devours the oxygen and the flames light up with dazzling brilliancy, but very soon the supply of oxygen fails, the light becomes less, when all at once, for no apparent reason, the burning wool throws out a dense mass of black flakes, which fill the jar in a thick cloud. The quantity of soot ejected would surprise any one but a chemist, as few would be aware that the unburnt liquid was capable of throwing out such a supply of carbon. It is needless to say that the effect here produced in the jar of oxygen is the same as that occurring in the chimney of a steam boiler when the supply of air is defective, the result being that so frequently seen, viz.: the pouring out of dense black smoke into the atmosphere. The loss of heat from unburnt gases may also take place without being made evident by the issuing of smoke. Thus carbonic oxide may pass away instead of carbonic acid. There have been a great number of inventions relating to the prevention of smoke in steam boilers which cannot be discussed in the space here available. The Smoke once formed cannot be burned. proper thing to do is to devise means to prevent the formation of smoke. The various modes in which fuel is wasted have been classified by Rankine somewhat as follows: I. Fuel is lost by the escape of gases in an unburnt state, or by permitting black smoke to be thrown off. Here the supply of air is defective, and the physical action is traced to the remarkable affinity of hydrogen for oxygen gas, whereby the |