Original Communication.

ON THE MEASUREMENT OF HEAT;

IN REFERENCE TO THE QUALITIES OF COALS.

BY ANDREW URE, M. D. F. R.S., &c.

(To the Editor of the London Journal and Repertory of Arts, &c.)

Sir,-An abstract merely of the following paper was read by me before the British Association for the Advancement of Science, at Birmingham :—

THE production of a like calorific effect denotes the agency of a like quantity or power of heat. Thus for example, when a pound of iron at the temperature of 50°, passes by any means to 51o, it has received the same calorific influence, whether the heat proceeds from the sun or a common fire-whether from the immediate contact, or from the radiation of a hotter body. Thus also, a pound of ice at 320 F, requires always the same quantity of heat to melt it, whatever be the circumstances of its liquefaction; and a pound of water at 2120 F, requires always the same quantity of heat to vaporize it, whether the evaporation be slow or rapid. On this fundamental principle, we can compare given quantities or powers of heat whenever we can apply them successively to produce the same effect; namely, to raise the temperature of a mass of matter, to liquefy a solid substance, or to vaporize a liquid. Since for this purpose, however, the heat must issue from the body in which it is contained, in order to pass into the body on which it is to operate a certain effect, it is evident that we can never compare the total or absolute quantities of caloric which bodies possess; for we can never exhaust all the caloric which they contain. Our measurement is restricted to those portions of heat merely which we can transfer from one body to another.

We say that a substance has more or less capacity for caloric, according as it requires more or less heat to suffer a given change of temperature, that of ten degrees of the thermometer for ex

ample and this quantity of heat is called the specific caloric of the substance. Its capacity is said to be constant, when with equal weights, equal quantities of heat are required to raise its temperature one degree at any point whatever of the thermome tric scale; that is to say, to make it pass from 50° to 51°, from 100° to 101°, from 1500 to 1510, &c. It is highly probable, that all solid and liquid bodies have a progressively increasing capacity; thus a pound of iron requires more heat to pass from 100° to 101o, than from 40° to 41°; and still more from 200o to 2010. The ratio of its capacities for two given points of the scale, as 32o and 212o for example, is the ratio of the quantity of heat which it requires at each of these points to undergo equal changes of temperature. In general, the ratio of the capacities of two substances is merely the ratio of their specific heats,—that is to say, the ratio of the quantities of heat which they respectively take in like weights and at the same degree, to suffer equal changes of temperature. It is usual to refer the capacities of different bodies to that of water, called unity or 1.00000. Thus, if the heat which raises water one degree, raises oil two degrees, we say that the capacity of water is double that of oil; or, if that of water be 1.000, that of oil is 0.500.

By duly considering these definitions, we may readily comprehend the methods and instruments which have been employed to determine the capacities or the specific heats of different bodies.

The first and most celebrated, though probably not the most accurate apparatus for measuring the quantity of heat transferable from a hotter to a colder body, was the CALORIMETER of Lavoisier and Laplace. It consisted of three concentric cylinders of tin plate, placed at certain distances asunder; the two outer interstitial spaces being filled with ice, while the innermost cylinder received the hot body, the subject of experiment. The quantity of water discharged from the middle space by the melting of the ice in it, served to measure the quantity of heat given out by the body in the central cylinder. A simpler and better instrument on this principle would be a hollow cylinder of ice of proper thickness, into whose interior the hot body would be introduced, and which would indicate by the quantity of water

found melted within it, the quantity of heat absorbed by the ice. In this case, the errors occasioned by the retention of water among the fragments of ice packed into the cylindric cell of the tin calorimeter, will be avoided. One pound of water at 172o F, introduced into the hollow cylinder above described, melts exactly one poundof ice; and one pound of oil heated to 1720 melts half a pound.

The method of refrigeration, contrived at first by Meyer, has been in modern times brought to great perfection by Dulong and Petit. It rests on the principle, that two surfaces of like size, and of equal radiating force, lose in like times the same quantity of heat when they are at the same temperature. Suppose for example, that a vessel of polished silver, of small size, and very thin in the metal, is successively filled with different pulverized substances, and that it be allowed to cool from the same elevation of temperature; the quantities of heat lost in the first instant of cooling will be always equal to each other; and if for one of the substances, the velocity of cooling is double of that for another, we may conclude that its capacity for heat is one half, when its weight is the same; since by losing the same quantity of heat, it sinks in temperature double the number of degrees.

The method of mixtures.-In this method, two bodies are always employed; a hot body which becomes cool, and a cold body, which becomes hot, in such manner that all the caloric which goes out of the former, is expended in heating the latter. Suppose for example, that we pour a pound of quicksilver at 2120 F, into a pound of water at 32°; the quicksilver will cool and the water will heat, till the mixture by stirring acquires a common temperature. If this temperature were 122°, the water and mercury would have equal capacities, since the same quantity of heat would produce in an equal mass of these two substances, equal changes of temperature, viz., an elevation of 90o in the water and a depression of 90° in the mercury. But in reality, the mixture is found to have a temperature of only 3710, shewing, that while the mercury loses 17410, the water gains only

510; two numbers in the ratio of about 32 to 1; whence it is concluded, that the capacity of mercury is of that of water. Corrections must be made for the influence of the vessel and for the heat dissipated during the time of the experiment.

The following calorimeter, founded upon the same principle as that of Count Rumford, but with certain improvements, may be considered as an equally correct instrument for measuring heat, with any of the preceding, but one of much more general application, since it can determine the quantity of heat disengaged in combustion, as well as the latent heat of steam and other vapours.

It consists of a large copper bath, e, f, capable of holding 100 gallons of water. It is traversed four times, backwards and forwards, in four different levels, by a zig-zag horizontal flue, or flat pipe d, c, nine inches broad and one deep, ending below in a round pipe at c, which passes through the bottom of the copper bath e, f, and receives there into it, the top of a small black lead furnace b. The annexed figure exhibits the structure of the calorimeter. The innermost crucible contains the fuel. It is surrounded at the distance of one inch, by a second crucible, which is enclosed in its turn by the sides of the outermost furnace; the strata of stagnant air between the crucibles serving to prevent the heat from being dissipated into the atmosphere round

the body of the furnace. A pipe a, from a pair of cylinder double bellows, enters the ash-pit of the furnace at one side, and supplies a steady but gentle blast, to carry on the combustion, kindled at first by half an ounce of red-hot charcoal. So completely is the heat which is disengaged by the burning fuel absorbed by the water in the bath, that the air discharged at the top orifice g, has usually the same temperature as the atmosphere.

In the experiments made with former calorimeters of this kind, the combustion was maintained by the current or draft of a chimney, open at bottom, which carried off at the top orifice of the flue, a variable quantity of heat, very difficult to estimate.

When the object is to determine the latent heat of steam and other vapours, they may be introduced through a tube into the top orifice g, the latent heat being deduced from the elevation of temperature in the water of the bath, and the volume of vapour expended from the quantity of liquid discharged into a measure glass from the bottom outlet c. In this case, the furnace is of course removed.

In my researches subservient to the calorimeter, which are still in progress, the first point to which I direct my attention is, the proportion of volatile and fixed matter afforded by any kind of fuel, as pit-coal for example, when a given weight of it is subjected in a retort or covered crucible to a bright red heat. The result of this experiment shews to what degree the coal is a flaming or gas coal, and what quantity of coke it can produce.

The second point is the heating power of the fuel, as measured by the number of degrees of temperature which the combustion of one pound of it raises, 600 or 1000 pounds of water in the bath,— the copper substance of the vessel being taken into account. My experiments have been hitherto directed chiefly to a comparison of the heating powers of Welsh anthracite, Llangennoek coal, and a few other coals. I have found that the anthracite when burned in a peculiar way, with a certain small admixture of other coals, evolves a quantity of heat at least 35 per cent. more than the Llangennoek does, which latter is reckoned by many to be the best fuel for the purposes of steam navi