1298. "An example will illustrate the application of this formula:-A piece of iron weighing 60 ounces, and at a temperature of 100° C., is immersed in 180 ounces of water, whose temperature is 19° C. after the temperatures have become uniform, that of the cooling water is found to be 22° C. What is the specific heat of the iron ?

1299. "Here the weight of the heated body, M, is 60; the temperature, T, is 100°; c is to be determined; the temperature of mixture, 0, is 22°; the weight of the cooling water is 180, and its temperature 19°; therefore180 (22-19). 60 (100-22)

c=

9

=

78

= 0·1153."

EXERCISES.

192. Taking the specific heat of iron as 0115, how much sensible heat would be absorbed by 10 lbs. of iron raised from 0° to 20° C. P

193. How much heat would be disengaged if 15 lbs. of copper were cooled down from 100° to 5° C., its specific heat being 0096 ?

194. If 3 lbs. of mercury, at 30° C., are mixed with 3 lbs. of water, at 10° C., and the temperature of the mixture is found to be 10'639° C., what is the specific heat of the mercury?

195. If 0.685 lb. of sulphur, at 60° C., is immersed in 4573 lbs. of water, at 12° C., and the temperature of the mixture is 13-42°, what is the specific heat of the sulphur?

1300. It is obvious that the vessel which holds the cooling water must change its temperature with that of the water, so that the heat lost by the substance which is cooled not only raises the temperature of the water, but also the temperature of the material of which the vessel is composed, to the same degree. In accurate experiments, the amount of heat required for this purpose must be estimated and allowed for. The amount is easily estimated, if we know the weight of the vessel, and the specific heat of the substance of which it is made. The vessel is usually a small cylinder of silver or brass, with

thin, polished sides, so that these data can be readily obtained. "If the weight of the vessel be m', and its specific heat d', its temperature, like that of the water, is t; consequently the previous equation becomes

Mc (T―0) = m (0 − t) + m' c' (0 — t)

from which, by obvious transformations,—

Generally speaking, the value, m' c', is put; that is to say, is the weight of water which would absorb the same quantity of heat as the vessel. This is said to be the reduced value in water of the vessel. The expression accordingly becomes

(m + μ) (0−t) "* c= M(T-6)

1301. In accurate experiments, it is necessary also to allow for the heat absorbed by the glass and mercury of the thermometer; and if, as is usually the case, the substance is enclosed in a glass tube in a small basket of wire-work, it is also necessary to pay regard to the weight and specific heat of these envelopes in the calculation. This method of determining the specific heat of solids and liquids admits of great accuracy, but its practical application requires many precautions, owing to the numerous sources of error.

EXERCISES.

196. A piece of iron weighing 20 grammes at the temperature of 98° C. is dropped into a glass vessel weighing 12 grammes, and containing 150 grammes of water at 10° C. The temperature of the water is thus raised 10-29° C. Required the specific heat of the iron, knowing that the specific heat of the glass is 0.19768.

197. Calculate the specific heat of oil of turpentine from the following data:-42 57 grammes of the oil at 33.7 were mixed with 470.3 grammes of water at 12-23°. The temperature of the mixture was found to be 15.57°;

* Ganot's "Physics." Edited by Dr. Atkinson.

the water was contained in a copper vessel weighing 45.25 grammes, and having a specific heat equal to 0·035.

1302. Regnault adopted this method in his very extended investigations on the specific heat of bodies. We shall give his own description of the last form of apparatus (Fig. 27) he employed. In the Memoir* in which he describes the apparatus, he states that he brings together the experiments he has made during several years for determining the specific heat of some simple bodies which he had never before succeeded in obtaining in sufficient quantity, and of a sufficient degree of purity. The methods which he employed for this new series of experiments differ little from those which he employed in his former experiments. He replaced the charcoal stove, which he employed in his former researches, by a gas lamp. This he found to be a great advantage.

1303. The substance submitted to examination is placed, generally, in a small basket of brass wire, M, provided in the interior with a cylinder of brass, in which is placed the bulb of the thermometer, T, which gives the temperature of the basket and its contents. The basket is suspended by a silk thread, which traverses the hollow metallic stopper, R. The thermometer, T, is maintained by this same stopper in such a manner that the division corresponding to 100° does not differ from the heat of the stove by a centimetre.

1304. The heated stove or steam bath is formed of three concentric cylinders of tin plate. The interior cylinder A, in which is placed the basket and thermometer, is hermetically sealed at the top and the bottom to the exterior envelope. Its superior orifice is formed by the hollow stopper R; its inferior orifice is closed by a moveable damper m, by which we heat the basket with the substance which it contains. Between the outer cylinder and the cylinder A is placed an intermediate one, which is fixed to the superior cover, and descends to the bottom of the conical part of the outer cylinder. The stove is maintained by a wooden support, DDPP', which serves besides as a screen for preventing the heat radiating upon the

"Sur la Chaleur Spécifique de quelques Corps Simples." Par M. V. Regnault.-Annal de Chimie et de Physique, 1861.

calorimeter H, where it occupies the place indicated in the figure, at the moment when we wish to immerse the heated basket.

с

1305. The boiler V communicates with the stove by the tube a b, which traverses the intermediate envelope, and brings in the vapour by the annular space B B, round the interior cylinder A. This vapour escapes by the openings 00, opening upon the opposite side to that by which the tube a b enters the vapour re-descends through the exterior annular space CC; the condensed water returns to the boiler V, by the lower tube c d. The vapour which has preserved the gaseous state escapes from the stove by the pipe ef, and is conveyed into the larger pipe G, terminating in the top by a narrower pipe, which is surrounded by cold water, which is being continually renewed. The vapour is completely condensed by this refrigerator, and returns to the boiler by the tube k p; moreover, it returns with a temperature very near 100° C., because in the space G it traverses the vapour which constantly arrives from the stove.

1306. The boiling of the water in the boiler V is produced by the gas lamp W; it continues to burn as long as is desired without requiring any attention; the same quantity of water serves indefinitely, as it returns without loss to the boiler.

1307. This new arrangement of the apparatus renders the operation extremely simple, and permits the experimenter to be occupied with other work.

1308. If we replace the water in the boiler V by other volatile liquids, we can obtain in the stove stationary temperatures very different from 100° C. Thus, if we employ, in the place of the water, sulphide of carbon, the temperature will be 46° C.; if we employ chloroform, the temperature will be 60°; with alcohol the temperature will be 78° C.; with turpentine the temperature will be 157° C.; &c. It is very easy to obtain in the stove a stationary temperature perfectly fixed; it suffices to place in the boiler V a liquid, the boiling point of which, under the ordinary pressure of the atmosphere, is little different from that which we wish to obtain in the stove; and then make this liquid boil under a greater or less pressure than the atmosphere, so that the thermometer of the stove is rigorously fixed at the temperature we wish to obtain. In this case we make the tube hi communicate, by an in