produced is 4300° to 4400°, when exactly this quantity of air is used. But if excess of air pass through the ignited carbon, or enter by any other channel, so as to mingle with the true products of combustion, it will share in the caloric generated. Suppose twice the requisite amount admitted, or 11.6 lb. in excess for 1 lb. carbon, what effect is produced?

In the gaseous current we now have,—

Carbonic acid, as before, 3-667 lb. at 2210 specific heat.

Nitrogen,

Air,

We may now find the heating effect of the caloric generated on this new gaseous mixture-and also its temperature.

Heating Effeci.-2631: 1.0000 :: 14,220° : 54,048, the temperature to which 1 lb. of the mixed gases would be raised by the heat generated in the combustion of 1 lb. carbon.

Temperature.-54,048° diffused over 24.2 lb. gives 2347°, which is the utmost temperature to which this new mixture can possibly rise; instead of 4347° with the minimum quantity

of air.

On this point we may remark generally,—

1. The caloric generated is neither increased nor diminished by excess of air. In all cases, it is capable of communicating 14,220° heat to cold water.

2. But this caloric is diffused through a greater quantity of matter when excess of air is used, and the consequence is a reduction of temperature as we increase the excess.

3. The degree of reduction may be found as above, by taking into account the increased quantity of gaseous matter in connection with the varying specific heat.

4. In the combustion of carbon, as the specific heat of air is nearly the same as that of the original products of combustion, the mean specific heats, with various degrees of excess, vary very little. The reduction of temperature is therefore almost exactly in proportion to the increase in weight. (This does not hold good in the combustion of hydrogen.)

NEW SERIES.-VOL. XI. NO. I.—JAN. 1860.

E

Application of the Heat from Combustion of Carbon to Praacticl purposes.

To present some features of this part of the subject at one view, Table III. has been drawn up. Its minuteness will obviate the necessity of repeating the calculations, and will also render more evident the mutual dependence of the various principles involved.

By 1 draught, or 1 equivalent of air or draught, we mean that no more air has been admitted than is necessary for perfect combustion-viz., 11.6 lb. per lb. carbon. 2 draught means 11.6 lb. in excess, and so on. We have assumed draught in excess from 1 to 2 equivalents rising by tenths, and from 2 to 20 by greater intervals, and it includes all air mingling with the products of combustion in any way.

Many persons do not seem to be aware of the extent to which excess of draught frequently exists in the ordinary use of fuel, and such may hastily conclude that the principles we advert to, and the formulæ founded on them, are inapplicable. We remark, therefore

1. It is well known that draught may be so increased that with a thin open stratum of fuel the fire will be at last extinguished.

2. The oxygen of the air must come into immediate contact with every particle of carbon before combination takes place; but it frequently occurs that parts of furnaces, more especially towards the fire-bridge, are not covered with fuel, and other parts so thinly as to allow the air to pass in streams.

3. Air is often admitted by openings above the fuel, with or without intention, at improper times. Such air is not at all required where the fuel is entirely carbonaceous,—such as coke &c., as the combustion is entirely confined to the fuel on the grate; and in hydrogenous fuels, it is required only during the period when the inflammable hydrogen compounds are being evolved, and not at all when the carbonaceous residue or cinder is burning alone on the hearth.

4. Many instances occur where the same amount of draught is maintained, whatever be the fluctuations of work done or fuel consumed.

5. The analyses of waste products by various observers show great excess in many cases.

In Table III., for every degree of excess of air, we have given the new mean specific heat (column 6); the heating power constant on cold water (8), but varying slightly on the increased products (9,; the pyrometrical effect, or utmost temperature attainable in each case (10); and the diminution of temperature per cent. (11). The most important points here involved are—

1. Absolute amount of Caloric generated.-As before remarked, this is the same in all cases where carbon is converted into carbonic acid by combustion in air. The various observers weighed the carbon, but not the air; and their results coincide. It must be remembered that they cooled down the products to their original temperature by ice or cold water.

This heat is capable of communicating 1° Fahr. to 14,220 lb. water per lb. carbon, but its value may be otherwise expressed, according to the purpose to which it is to be applied.

Expressed in pounds water raised from freezing to boiling,

32° to 212° 180°. 14,220 ÷ 180 = 79 lb.

Raised from mean temperature to boiling point,

60° to 212° 152°. 14,220 ÷ 152 =

Raised into steam from mean temperature,-

60° to 212° = 152° Latent heat in steam, 965°

}

93 lb.

1117°. 14.220°÷ 1117 = 12-73 lb.

Raised into steam from boiling point,

Latent heat in steam, 965°. 14,220 965 1474 lb.

2. Utmost Temperature, or Pyrometric Intensity.-In column 10, this is given for every degree of draught. The mode in which it is determined has been already indicated.

This temperature can be expected in practice only when all the heat produced is present in the gaseous current, at the point where we look for such temperature. In most cases part of it is immediately radiated and absorbed, as in steam boilers where the furnace is inside or immediately under; but

where the fire is surrounded by non-conducting material, such as brick, the temperature in the space immediately over the fuel may be expected to agree generally with the Table.

An inspection of the Table will show, that with every addition, however small, of air in excess, the utmost temperature attainable decreases. For instance, the current of hot gases rising from a fire of charcoal with a draught of 2 equivalents, or double that requisite for perfect combustion, would indicate about 2000° were a pyrometer freely suspended in it.

By reference to column 6, it will be seen that the mean specific heat of the current varies only slightly as we increase the proportion of air. The diminution of temperature is therefore almost exactly in proportion to the increase of draught, and we are thus able to reduce the matter to a formula or rule easily remembered.

Formula 1. To find the initial temperature due to any amount of air or draught, divide 4400° by the equivalents of air.

The results by this formula coincide closely with column 10. Example.-Required the initial temperature where 14 equivalents of air are given, or a draught of 17 lb. air, per lb. carbon, instead of 11-6 lb.? 4400 1.5 2933°. With double draught? 4400°÷ 2 = 2200°. With ten-fold draught? 4400° 10 = 440°.

By the term "Initial Temperature," as used above and subsequently, we mean the temperature of the gaseous current arising from combustion, after all the atmospheric air from any opening whatever has been added to it, and before any of the caloric has been abstracted.

This formula applies, without any material qualification, to the use of all purely carbonaceous fuels, such as coke, wood, charcoal, anthracite, &c.; and as the laws of nature are inexorable, we must suit our modes of procedure to their requirements. We will afterwards see the important bearing of initial temperature on economy of fuel, extent of heating-surface in boilers, &c.

To the use of common coal or wood in its natural condition, this rule also applies; with a qualification, however. Immediately after fuelling, and while the volatile hydrogen and

hydrocarbons are being evolved, air ought to be admitted above the fuel, or through the fuel in excess, because these volatile elements burn in the furnace, and not on the hearth. And while this goes on, the initial temperature due to the carbon will be modified by that due to the hydrogen, which is about one-fifth less for the same proportion of air. But so soon as the hydrogen element disappears, we have carbon alone, in the shape of charcoal, coke, or cinder, and at this stage all fuels become amenable to the law under consideration.

In many applications of fuel, intensity of temperature is required, as in metallurgic operations, the manufacture of alkalies, &c. Cast-iron melts at a temperature of 3000° to 3300; a draught of 1.5, producing a temperature of 2951°, could not, however long continued, melt a single grain of that metal. 10 draught might boil water, but not melt lead; 5 draught, giving 908° temperature, might melt lead, but not copper.

In all such cases, the quantity of fuel does not decide the temperature, or determine the effect; all depends on the relative amounts of fuel and air. A very small opening in the door or elsewhere of a reverberatory furnace may cause much loss of time and fuel, by its paralyzing effect on the temperature of the gaseous current.

3. Amount of Heat carried off by the "Waste" Products of the Combustion of Carbon.-By "waste products," we mean the gaseous current after it has been applied to heating purposes.

As before remarked, carbon generates 14,220° heat; but to obtain it all, we must cool the gaseous current to its original temperature that of the atmosphere. In the testing experiments by Andrews and others, this was done, but is not practicable in the ordinary use of fuel. The departing current must of necessity be, in all cases, as warm as, and is in general much warmer than, the surface or body to which it has communicated caloric. This involves loss of heat; and we will now consider the laws which regulate the loss under varying circumstances, and draw such general conclusions as may be practically useful.