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, pound of fuel.

Thirteen cubic feet of air, at 60° Fah. and under a pressure of 30 inches of mercury, weigh abont I 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 aid.

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

oxygen is absorbed to the exclusion of carbon in the first instance.

2. There is waste from external radiation and conduction. M. Peclet states that the quantity of heat radiated from incandescent charcoal is 5 per cent. of the total heat of combustion, and that the heat radiated from coal somewhat exceeds that radiated from charcoal. The practical conclusion to be drawn from this statement is, that the heat radiated from the burning fuel should be carefully intercepted in every direction.

Hence the economy resulting from the use of an internally fired boiler with internal furnace tubes.

As to the heat radiated into the ash-pit, that is carried back again to the fire by the current of entering air. In respect of the loss of heat by conduction, that is obviated as much as possible by the use of fire-brick; and where the furnace is outside the boiler, the resistance to conduction is increased by double layers of brickwork with enclosed air spaces between the layers.

3. There is loss of heat by the escape of gases up the chimney at a temperature above that which is necessary for maintaining the draught.

A general idea of the value of a chimney in promoting the draught of a fire may be gathered from a statement of a law which appears to be approximately true, viz: That the velocity of air,

as due to increased pressure, is that acquired in falling down a height equal to the uniform column which gives the increased pressure.

In making any calculation on this subject it is usual to adopt the hypothesis that air is incompressible and behaves as a liquid.

Let the increase of pressure support 5 inches of water. We know that 29.922×13.596 inches of water balance the pressure of the atmosphere which would be produced by a stratum of incompressible air 26,214 feet high.

Therefore, I inch of water will balance 64.4 feet of air.

Hence 5 inches of water balances 322 feet of air: therefore velocity due to increase of pressure

=

√64.4X322

= 64.45

=

= 144 feet per second, very nearly.

According to the old rule the area of the chimney should be that of the fire-grate, and there should be 1 square foot of fire-grate for each horse-power.

Rankine gives formula for computing the height of a chimney in order to produce a given draught, and states that the best chimney draught takes place when the absolute temperature of the gas in the chimney is to that of the external air as 25 to 12, or when the density of the hot gas is one-half that of the external air.