that fuel, burned in pure oxygen, evolves the same amount of heat as when perfectly burned in atmospheric air. The experiment is conducted in the following wayThirty grains of the finely-powdered sample of coal, or other fuel, are very intimately mixed with from 300 to 360 grains of a mixture composed of three parts of chlorate of potash to one of nitrate of potash. The mixed mass is then introduced into a copper cylinder column, which forms the furnace. This is then placed on a brass base, and a hole is made in the mixture to receive a fusee (a little cotton wick which has been dipped in nitre, and which serves as a slow match), about half an inch in depth, which is fixed by pressing the mixture around it. A copper cylinder, with a long copper tube fitted with a stop-cock, is called the enclosure. The copper cylinder is perforated with a number of small holes to allow the gases produced by the combustion to pass through the water, and thus give up their heat to it. The copper cylinder is fitted over the furnace, and when the furnace and cylinder, thus joined, are introduced into water, the water is excluded or admitted to the furnace according as the stop-cock is shut or opened. A glass cylinder, filled with water up to a certain mark, holds 29,010 grains. The temperature of the water is determined by a very delicate thermometer. It ought to be about 60° F. at the commencement of each experiment. The temperature of the water having been determined, the fusee is lighted, and the enclosure, with its stop-cock closed, is fixed over the furnace. The whole apparatus is then submerged in the water contained in the glass cylinder. A minute or so after the immersion, the wick burns down to the mixture, its combustion then commences, and when it ceases, the stop-cock is opened, and the apparatus (furnace and enclosure) moved gently up and down in the water, which causes the fluid within and without the enclosure to acquire the same temperature. The increased temperature of the water_is then determined by means of the thermometer. The number of degrees to which the water has been heated, represents the quantity of water which would be con verted into steam from a temperature of 212° F. If, for example, the temperature of the water has been raised 10°, we learn that the sample of fuel under examination

would be capable of converting ten times its weight of water into steam, since every grain of the fuel has been burnt in the midst of 967 grains of water; and if the latent heat of steam be taken at 967° F., then if 967 grains of water be raised 10°, sufficient heat has been generated to boil off 10 grains of water from 212° F.: substituting the word pounds for grains, we have at once the evaporative value of the coal.

1408. As part of the heat generated by the coal is absorbed by the instrument, this per-centage must be added to every calorific result obtained.*

1409. On the burning of fuel.-Owing to several causes,

"The absolute thermal effects of different fuels,-different combustibles, -are represented in this table, and you will there see the great preeminence of hydrogen in this respect. Taking the absolute thermal effect of carbon as unity, the same weight of hydrogen produces in its combustion nearly 4 times the heat of the carbon; and olefiant gas 14 times, and so on. Here is the table:→→

Absolute thermal effect of combustibles.

The absolute thermal effect, ascertained by experiments of this kind, of different kinds of fuel in ordinary use, especially of the two kinds of fuel generally used for heating purposes, in contradistinction to gas, is shown in this table.

"Quantity and cost of gaseous and solid fuel required to evaporate 1,000 lbs. of water from 2120:

1,833 cubic feet of coal gas cost, at 4s. per 1,000 cubic feet

2,874

"

125 lbs. coke

112 lbs. coal

water gas

at 28.
at 20s. per ton
at 20s.

7s. 4d.

58. 9d.

1,833 cubic feet of coal gas, at 4s. per thousand, are required to evaporate 1,000 lbs. of water from 212°. To do this amount of work, therefore, you are required to employ a quantity of gas which costs 7s. 4d. If, in the place of coal gas, you use gas which is produced from water by charcoal,-a mixture of hydrogen and carbonic oxide chiefly,-2,874 cubic feet are necessary, which will cost the sum of 58. 9d. for that amount of heating power: 125lbs. of coke will do precisely the same amount of work-will heat exactly the same quantity of water to the same temperature-for ls. 1d., or 112 lbs. of coal will do the same for ls. I have put this diagram before you to show at a glance the great difference there is between these kinds of fuel. You see that coal and coke produce a much greater amount of heat at a less cost than gas, although gas has been proposed for heating purposes; and no doubt there are great facilities for the use of gas which may contribute, to some extent, to counterbalance the great discrepancy in its cost."— Lectures delivered by Dr. Frankland before the Royal Institution. See "Chemical News," vol. iii., 1861.

the total theoretical heating power of fuel is never ob tained in practice. 1st. The fuel is scarcely ever completely consumed, a part escapes combustion by passing off in the form of combustible gases and smoke, and another part remains mixed up with the ash. 2nd. There is loss of heat from radiation, and also from conduction; the loss by conduction not only occurs through the matter form. ing the furnace, but also by means of the gaseous current which is constantly flowing through the incandescent fuel. In calculating the calorific intensity, the theoretical amount of air required for the combustion of the fuel is employed, but in practice this is, generally speaking, never accomplished, for it requires, in order to approach theory, the most favourable circumstances, such as properly arranged furnaces, the adjustment of the fuel, and draught of air passing through the fire; this latter requires skill and constant attention on the part of the fireman; generally too much air is allowed to pass through the fire, which increases the loss of heat; and the loss from this cause is greater than is generally supposed.

66

1410. In calculating the theoretical temperature it is presumed that the oxygen of the air is in contact with every particle of the fuel during ignition. Could the same condition be insured in the furnace, it is evident that the great desideratum required in combustion would be attained; no escape of combustible gas could then ensue, nor of the carbonaceous particles which give to the gases passing up the chimney the character of smoke; and the greatest possible heat arising from the burning of the substance in air would be developed. Indeed, nothing would be wanting to extract from the fuel the benefit of its total theoretical heating power, but such an arrangement of the furnace as would perfectly utilize the caloric so produced. No one, however, who has any experience as to the manner in which the fire is managed in the ordinary kinds of furnaces, will hesitate to assert that the above conditions are never supplied."

1411. The time required for the combustion of the fuel, and, consequently, for the evolution of heat, depends upon its state of division and aggregation, and also upon its chemical composition. If the fuel be thrown on the

* Muspratt's "Applied Chemistry."

fire in large pieces, it burns slowly, and a large proportion of the heat generated is absorbed. If it be wood that is employed, and instead of being burnt in the form of large logs, it be first divided into shavings, the combustion will be so rapid that a large proportion of the heat will, for all useful purposes, be lost. This arises from the greater facility with which the air comes in contact with it, when in the form of shavings. If, however, the fragments are still further reduced in size, the smallness of the particles, and the close contact existing between them, excludes the entrance of the necessary supply; and for this reason it is extremely difficult to obtain any available heat, either from sawdust, or very finely-divided coal. But in determining the value of a fuel, not only must the state of division, but also the state of aggregation be taken into account; thus, "particular qualities of charcoal, coke, and anthracite may have the same calorific power, and yet differ remarkably in their manner of burning. Of the three, charcoal, being very light and porous, ignites most easily, and in a given volume contains the least combustible matter; and accordingly, under the same conditions, it is most quickly consumed. Coke also contains less combustible matter in a given volume, and, except when prepared at high temperatures, is more easily ignited than anthracite. The practical effect of these differences in the manner of burning will be well understood by experimenting on these three kinds of fuel in a common casting furnace about one foot square and from two to three feet deep. If an attempt is made to heat a large crucible in such a furnace by means of anthracite, it will be found that the bottom becomes heated to whiteness before the top is hardly red-hot; whereas, by the use of coke, the temperature is not so excessive at the bottom, but is more equally diffused through the furnace. The effect of anthracite as a fuel, is the rapid production of an intense heat confined to a space not extending beyond a few inches above the bars."*

It is obvious that, on this account, anthracite is not adapted as a fuel for ordinary steam-boiler furnaces; but by the following simple contrivance, it may be advantageously employed in these furnaces. The ash-pit is kept filled with water, and deep, fish-bellied bars are used, of which the lowest parts nearly, if they do not actually, touch the water. Steam is necessarily evolved from the surface of the water, and enters the fire place along with the atmospheric air which sustains combustion. On passing through the

1412. The student will have learned from the exercises that the more carbonaceous the fuel, the greater its calorific intensity; hence charcoal, coke, and the highly carbonaceous coals of South Wales, are well adapted for purposes where a high localized temperature is required, as in the smelting of iron; but for steam purposes, where a more diffused heat is required, and where, especially, the steam is required at intervals, a coal containing more hydrogen will be better adapted for the purpose.

CHAPTER XIV.

DIFFUSION OF LIQUIDS-DIALYSIS-OSMOSE.

Introduction, 1413. Characters of liquid diffusion, 1423. Diffusion of various salts and other substances, 1428. Diffusibility of acids, 1435. Diffusion of mixed salls, 1447. Separation of salts of different bases by diffusion, 1448. Decomposition of salts by diffusion, 1449. Diffu sion of double salts, 1456. Diffusion of one salt into the solution of another salt, 1461. Diffusion of salts of potash and ammonia, 1467. Crystalloid and colloid substances, 1470. Effect of temperature on diffusion, 1483. Diffu sion of crystalloids through colloids. 1484. Dialysis, 1489. Distinction between crystalloids and colloids, 1496. Appli cation of dialysis, 1507. Osmose, 1527.

1413. In this chapter I have endeavoured to place before the student a full and accurate abstract of the researches of Mr. Graham on liquid diffusion; this ab

incandescent anthracite, it is decomposed, with the formation of the com bustible gases, carbonic oxide and hydrogen, which are afterwards burned under the boiler at a distance from the fire, by the admission of a suitable supply of atmospheric air from without. The decomposition of the steam causes a considerable diminution of temperature within the fire place, but there is no permanent loss of heat, as, on the subsequent burning of the combustible gases derived from the steam, the heat absorbed in the first instance is again given out and economised; there is, so to speak, only a transference of heat from the fire place to a distance. The bars do not become sufficiently heated to burn rapidly away. The fire place should be enclosed above by a fire-brick arch, as no part of the boiler should be unprotected above the solid fuel."-Dr. Percy's "Metallurgy.”