The coals annually raised in Great Britain have been estimated at not less than 35,000,000 of tons, of which not more than 3,000,000 are exported,leaving for home consumption 32,000,000 tons of various kinds. Nearly 3,500,000 tons of this quantity are annually consumed in London! Of iron we raised in 1750 only 30,000 tons; by 1850 the quantity had increased to 2,250,000 tons. According to the official returns of 1849, the export of iron of all kinds was valued at £4,500,000.

Artificial Fuel.-The term artificial fuel has been applied to signify various combinations of combustible matters, either of which alone would present mechanical or chemical conditions unfavourable to combustion for practical appliances. The first attempt at the manufacture of this kind of fuel appears to have been undertaken in Norway, where the sawing of large quantities of timber gives rise to a quantity of sawdust, not adapted to the purposes of fuel in its original state, but which, nevertheless, contains similar elements of combustion with wood itself. Accordingly, the sawdust was mingled with tar; and being pressed into blocks, these were applied to furnace operations.

We in these isles have not a sufficient accumulation of sawdust to render its employment as fuel a desideratum; a remark which equally applies to Germany. Both ourselves and the Germans, however, have enormous magazines of turf, and wasteful debris of small coal, both of which it would be desirable to convert into the mechanical shape and condition subservient to good combustion. Accordingly, numerous kinds of artificial fuel, consisting of burning turf, turf-charcoal, and small coal, have been prepared, with various degrees of success. Swazil, an Austrian, combines porous turf with organic matter, induces a peculiar state of decomposition between the two, and generates, it has been said, an effective artificial fuel. Hill, a British chemical manufacturer, first submits turf to destructive distillation, thus libe rating tarry products, which he subsequently incorporates with the charcoal left behind, and, by pressing the compound result into blocks, in this manner prepares his fuel. Another inventor, a foreigner, mixes small coal with fat, and presses the result of the mixture into blocks. Warlich mixes small coal with alum or salt, forms the mass into blocks, and exposes them to destructive distillation in a retort. Bessemer's process of artificial fuel manufacture is based upon the circumstance that small coal, when heated to about 500° or 600°, becomes pasty, and agglomerates. Such are the chief varieties of arti ficial fuel-neither of which is so much a desideratum with smelters, as appl:cable to the purposes of steam-ships; while condensation of combustible matter and the facility of stowage, dependent on regularity of form in the masses, are matters of especial consequence.

Comparative Value of Fuel.-Pyrometrically deduced, this is a most important subject of inquiry to the metallurgist, and it is one beset with many difficulties. We shall indicate the principal methods of calorific estimation which have been devised, repeating, however, the observation already made, that for practical operations the mechanical structure of a fuel is a consideration of almost equal importance with its chemical nature.

Inasmuch as there is no absolute or naturally existing want of heat, the philosopher, in prosecuting his researches in this field, is limited to the consideration of the amount of heat by which a given quantity of one combustible exceeds the amount yielded by the same quantity of another. In making this comparison, we may compare equal weights, or equal measures; if the former, we arrive at the absolute-if the latter, we deduce the relative effects of heat. Firstly, we shall review the principal methods which have been devised for acquiring a knowledge of the absolute effects of heat. And first, by the method proposed by Rumford-premising that, in expressing calorific effects in thermometric degrees, the Centigrade scale will be adopted, as affording greater facilities of calculation than the scale of Fahrenheit.

Rumford's method of estimating the absolute effects of heat developed by a fuel consists in determining the quantity of such fuel necessary for heating a given weight of water from 0° to 100°C. According to this mode of investigation, it is found that, respectively, equal weights of hydrogen, pure carbon, and wood-charcoal heat water from 0° C. to 100° C. in the following ratio :

Reflecting now that the number of degrees in the Centigrade scale between 0' C. and the boiling point of water is 100, it follows that the respective figures of absolute heating power for each degree, and for each of the bodies respectively above-mentioned, may be deduced by multiplying the parts by weight of water heated from 0° C. to 100° C. by 100: whence it follows that

Thus we deduce what may be termed the respective unities of heating effect for hydrogen, carbon (pure), and wood charcoal. Furthermore, the above relations admit of simplification. Natural unit of absolute heat, we have already mentioned, there is none. It will be desirable, therefore, to assume a conventional or empirical standard; and carbon being, par excellence, the combustible involved in technical operations based upon combustion, the substance may be taken as the conventional unit. Dividing, therefore, the first and the last of the numbers given in the preceding series, by the middle number, i.e. 7800, as that indicating the unit of absolute heating effect for carbon, we then make carbon the prime standard or unity, representing its absolute heating effect by 1, and the respective absolute heating effects of hydrogen and wood-charcoal as represented by the following series :—

Carbon
Hydrogen.
Wood-charcoal

From this statement the deduction is arrived at, that the more hydrogenous a fuel is, the greater the absolute heating effects resulting from its combustion.

Next, we shall consider the process devised by Karmarsch for obtaining similar information. Instead of measuring the absolute heating power of a combustible by ascertaining the weight of water it was capable of heating from 0° C. to 100°, Karmarsch ascertained the weight of water which a given weight of fuel during combustion could raise in steam. This was the process followed by Brix, in determining the relative value of Prussian fuels. It is, perhaps, unnecessary to indicate, that, to arrive at the greatest correctness of which the process is susceptible, the water used in the series of comparative experiments must be taken at the same temperature. Brix, in conducting his investigations, used water of 0° C. or 32° F.: his scientific expressions were, moreover, conveyed in terms of the Reaumur thermometric scale.

Berthier's method is more complex. It is based upon atomic, or (to avoid the language of theory) equivalent proportional considerations, which the following remarks will, it is presumed, make evident. Inasmuch as the equivalent weights of hydrogen, carbon, and oxygen, are respectively 1, 6, and 8, and the respective compositions of water and carbonic acid are

it appears that two parts by weight of hydrogen, in generating water by combustion with oxygen, consume sixteen parts of the latter; whereas the same quantity of oxygen (i.e. sixteen parts by weight) is only consumed by six parts of carbon, in becoming carbonic acid. Hence the deduction follows, that a given weight of hydrogen in forming water by combustion, unites with three times as much oxygen as an equal weight of carbon during the combustion of the latter, and the formation of carbonic acid. Now, the fact has already been proved as a result of Rumford's mode of experimenting, that the relative amount of absolute heat developed, as between hydrogen and carbon, is as 3 to 1; whence it clearly follows, that the absolute heating effects of carbon and hydrogen are in direct proportion to their combustive capacity of absorbing oxygen: a deduction, indeed, first promulgated by Welter, and which— though since his time it has undergone modifications-is still reliable. Berthier expressed the absolute heating effects of any given combustible, in terms of the weight of oxygen consumed, by a given weight of the combustible during the progress of its combustion. This was practically effected by heating the combustible in contact with oxide of lead. It follows that, in proportion to the quantity of oxygen taken away, so would be the weight of lead reduced to the metallic condition; and this latter being collected and weighed, the absolute heating effects of the combustible were expressed fractionally by the weight of oxygen consumed, but directly in terms of the lead reduced. In this way he found that,

If it be now assumed that any particular combustible under examination should be in relation to a mass of lead, the weight of which is m, then the ratio between the absolute heating effect of the combustible in question, as com

34

m

pared with that of pure carbon, will be expressed by the fraction = and the statement for this absolute heating effect expressed in units of heat will be m =78.100 34

=230.m

Though somewhat complex in theory, the process just described is conducted with facility. The results are tolerably reliable; but they involve a constant error of about th. Forchammer has somewhat improved this process, but the results still involve a constant error.

The chemical reader will perceive that the process of Berthier just described is very nearly allied to the ordinary combustive operations followed in organic analysis. If, instead of oxide of lead, oxide of copper be employed, we have at once the preliminary conditions of ultimate organic analysis. Accordingly, this means of estimating the absolute heating effect of fuel has been frequently adopted, the oxygen appropriated by combustion being calculated in the usual manner from the amount of carbonic acid and water respectively developed, and the final reduction into terms of absolute heat being deduced from the application of Welter's laws, explained in page 47.

Besides the methods already indicated, may be noticed the process described by Sheerer in his Metallurgie, vol. i. p. 139, founded on consideration of the chemical composition of the fuel. In addition to which, processes have been devised with the same object in view, by Laplace, Lavoisier, Dalton, Marcus Bull, and others.

Specific Heating Effects of Fuel.-We have already explained that if fuels be compared as regards the respective thermic effects of their combus tion, weight for weight, the respective absolute heating effects of each will be deduced; whereas if they be compared, bulk for bulk, then we arrive at the specific heating effects of each. We arrive, therefore, at the following deduction:-Knowing the absolute heating effect of any combustible, we arrive at the knowledge of its specific heating effect by multiplying the numerical exponent of the former by the number expressing the specific gravity of the combustible.

Such, then, are the indications of theory and the results of laboratory operations thereupon founded. The practical smelter need not be told that not even a near approach to the figures arrived at is possible on the large scale. Such accordance between theory and practice could only result if the total amount of heat generated by the combustion of a fuel were concentrated on the ore submitted to its agency. This is an impossible condition; even more heat invariably escapes from furnaces in the state of combustible gas and as radiant heat, than the actual amount of heat made effective. Much may be accomplished in lessening the divergency between theory and prac

PYROMETRIC EFFECTS AND PYROMETERS.

49

tice by drying the fuel, as much as circumstances will permit; by regulating the amount of air-draught, applying the hot gaseous products of furnace-combustion to subsidiary purposes, and taking advantage of other conditions which, though they vary in different establishments and for different metallargic operations, depend on a few unvarying scientific principles, and will readily suggest themselves to an intelligent observer.

Pyrometric Effects of Different Fuels.-The amount of heat evolved by any given fuel, whether in relation to its weight or its bulk, differs widely from the degree of heat capable of being yielded by it in any one locality. Varying the conditions involved a little (though not fundamentally) by assuming a parallel case, evidently a quart of boiling water contains twice the amount of heat which a pint of boiling water, though the contents of each will cause the thermometric column to stand at 212° F. Now 212° F. is, therefore, said to be the thermometric heat of boiling water; and if a thermometer could be procured capable of indicating furnace heats, we should arrive at deductions parallel with that obtained from the consideration of boiling water. For the estimation of combustive heat, however, special methods must be had recourse to; inasmuch as thermometers are not competent to indicate temperature of such high degree. The deductions are arrived at, either directly by the aid of instruments (pyrometers), or indirectly by calculation.

It is evident that the principle of expansion of liquids in tubes is unadapted to pyrometric construction, inasmuch as the utmost limit of the indication of such instruments can only fall somewhere within the boiling point of the liquid employed. Gaseous and solid bodies are, therefore, alone adapted to the construction of pyrometers.

Pyrometric estimations admit of division into three general classes: (A) those the indications of which are based upon the expansion of some particular body (pyrometer); (B) those which are founded on a consideration of the heat imparted to water by a heated body; and (C) those the results of which are deduced from considerations of the melting points of metals and metallic alloys.

(A.) Pyrometers. In the year 1782, Wedgwood invented the pyrometer which has since borne his name. Its agency depends on the contraction of clay experienced when exposed to high temperatures. A cylinder of clay of some definite size when cold, which shrinks before heating to a known extent between two converging sides of a wedge-shaped cavity, sinks lower down in the same cavity after having been heated; and Wedgwood assumed that the amount of contraction of the clay would be always proportionate to the degree of heat to which it might have been exposed. This, however, is a false assumption: a long-continued and moderated heat is found, in practice, to cause an equal amount of contraction, or shrinking, with a more violent degree of heat during a shorter period; hence it follows that the indications of this instrument are all but worthless.

Daniell, Guyton Morveau, Neumann, Peterson, and Gibbon, and some others, have devised each respectively a pyrometer, the general principles

USEFUL METALS.