ABCDE Sieve for sifting fuel. Bottle containing oxygen mixture. F for Brass Base to support E and G. K Condenser Cylinder. Glass Cylinder filled with 1934 grammes water. Wire pricker used in clearing the tube, G. L for Spatula, used to mix the fuel and oxygen mixture. factured by Messrs. Alex. Wright & Co., London. This apparatus has been adopted by several Railway Companies, as well as by the British and Foreign Governments, &c., to determine the relative commercial value of the coals which they purchase. Principles on which the Calorimeter is Based-First, that the latent heat of steam is equal to 9666 (or in round numbers 967) British Units of heat; and secondly, that coal burned in pure oxygen evolves the same amount of heat as when perfectly burned in atmospheric air. Thus it follows, that if we heat or raise 967 parts (by weight) of water 1° Fah., we have employed as much heat as would have boiled off 1 part (by weight) of water from 212° Fah. If the same quantity of water had been heated 10° Fah., then we have used heat equivalent to boiling off 10 parts of water, and so on. The thermometer being used to indicate in this way, the number of parts of water capable of being boiled off by the heat generated by the perfect combustion of the fuel. As one pound of coal is an inconveniently large quantity of fuel to use with such an apparatus, it has been specially designed for burning 2 grammes of fuel at a time. From the above it will be observed that, if we contrive to thoroughly burn 1 gramme of fuel in the midst of 967 grammes of water, the increased temperature of the water will show us the number of grammes of water which 1 gramme of the fuel in question is able to convert into steam at 212° Fah., and we have thus at once the evaporative value of the fuel. As it has been found advisable to use a larger quantity of coal or other fuel than 1 gramme, the apparatus is specially designed (as we have just remarked) for 2 gramme quantities, and of course for a corresponding quantity of water-viz., 967 x 2 or 1934 grammes. As the weights of fuel burned and of water used are directly proportional to the latent heat of steam, the indications of the apparatus are general, and may be expressed in any weights whatever-e.g., in pounds of water boiled off per pound of coal. Instruction for Taking the Test.-(1.) Select from a mass of the coal or fuel to be tested several portions which will collectively represent an average sample, and reduce them to a powder. Mix the samples thoroughly, and put about 20 grammes of it into the iron mortar, A, and pound it well. Then sift it through the sieve, B, on to a sheet of white paper. Return the coarser particles to the mortar, and pound them again, as well as sift them, and intimately mix the powdered fuel. (2.) The oxygen mixture for combustion of the fuel is kept in the bottle, D, and consists of 3 parts of chlorate to I part of nitrate of potash, each of the purest quality, reduced to a powder, so that it will pass through a gauze of 1000 meshes to the square inch, which powder must be perfectly dry. Two grammes of the powdered fuel are now intimately mixed with from 8 to 13 (or more) times its weight of this oxygen mixture, by means of the spatula, L. (3.) Carefully introduce the resulting grey powder into one of the copper furnaces, E, and compress it down in successive small portions, by means of a rammer or test tube, but without tapping, for if tapping or shaking be resorted to the rougher portions of the oxygen mixture rise to the surface. (4.) The furnace, E, is next placed in the socket of the brass plate, F, and about inch length of fuse placed on the top of the powder as seen in the figure at, F. (5.) Fill the glass cylinder, H, up to the engraved line with water (i.e, put in 1934 grammes), and take the exact temperature of the water by means of the delicate thermometer, J. All thermometer readings must be read to at least one-tenth of a degree, otherwise the exactness of the experiment will be impaired. It is important to bring up the temperature of the water to within a few degrees of that of the room by adding if necessary a little hot water, and extracting any excess of water above the 1934 gramme mark by means of a pipette. The following relative temperatures are recommended : (6.) Having the Condenser Cylinder, G, with its stop-cock closed, ready at hand, light the fuse of the charge on, F, and at once cover it with the condenser, pressing the latter firmly down upon the clutch springs of the base, and immediately submerge the whole in the water contained in the glass cylinder, G. This whole operation must be performed before the fuel and oxygen mixture is ignited, otherwise the whole would be consumed in the air, and the experiment destroyed. See central figure, "Calorimeter in Action." (7.) When combustion has ceased, open the stop-cock of the condenser, and if air does not freely issue insert the long wire, K, and move the condenser several times up and down in the water, to cause the water within and without the condenser to assume the same temperature. Blow away the smoke, and note the temperature of the water, losing as little time as possible. As an average of 10 per cent. of the heat generated by the fuel is absorbed by the instrument, add 10 per cent. to the observed temperature. (8.) The furnaces must be well cleaned and dried out before being re-used. TEST WITH A WELSH STEAM COAL, BY F. W. HARTLEY, A. INST. C.E. Water raised in degrees Fah. + 10 per cent. for instrument, .. Weight of water that would be evaporated at 212° F. per pound of such coal, if the water received all the heat from its perfect combustion, British heat units generated per pound of the coal grammes x 14°4÷2 grammes of fuel,. 26 65 14.4 14.4 lbs. * = 1934 13,925 *The same result was obtained by Prof. Sexton of the College of Science and Arts, Glasgow, from the locomotive steaming coal used by the North British Railway Company, March, 1887. LECTURE IV.-QUESTIONS. 1. What do you mean by the quantity of heat in a body, and how is it measured? 2. What is the unit of heat adopted in Great Britain? How many units of heat are imparted to a cubic foot of water (62 5 lbs.), on raising it from 60° to 212° F., also to 1 lb. of copper? Ans. 9,500, and 14:44. 3. Define and show the difference between the terms capacity for heat" and "specific heat" of a substance. Suppose a substance was given to you to find its specific heat, how would you conduct the experiment? Give an arithmetical example. 4. If 1 lb. of platinum is plunged into 1 lb. of water at 50° F., and the resultant temperature of the water is 112° F., what was the original temperature of the platinum? Ans. 2,025.5° F. 5. If 2 lbs. of copper at 500° F. are plunged into 4 lbs. of water at 50° F., what will be the resulting temperature? Ans. 80° F. 6. Define "specific heat." Deduce a formula for determining the relation between the masses, specific heats, &c., when two substances are mixed together. A piece of platinum, weighing 1 lb., is suspended in the hot gases of a furnace whose temperature has to be ascertained. After being heated to the temperature of the furnace it is taken out and plunged into 2 lbs. of water at 49° F. The resulting temperature of the mixture is found to be 100° F. Determine the temperature of the furnace, having given specific heat of platinum =0034. Aus 3,100 F. LECTURE V. CONTENTS.-Transfer or diffusion of Heat-Radiation-ConductionConvection. Transfer or Diffusion of Heat. It was explained in the last lecture, that equality of temperature between two bodies exists, when there is no tendency to a transfer of heat from either to the other. We saw also that, when their temperatures differed in the slightest degree, there is a tendency to an equality of temperature, by a transfer of heat from the hotter to the colder, and that this tendency is greater, the greater the difference of temperature between the bodies. Rankine states that the rate at which the transfer of heat takes place between two bodies, at unequal temperatures, depends"First. On the tendency to transfer heat, increasing as some function of the two temperatures and their difference. "Secondly. On the areas of those parts of the surfaces of the bodies through which the transfer of heat takes place. In most of the cases which occur in practice, those areas are equal, and then the rate of transfer of heat is directly proportional to their common extent. Thirdly. On the nature of the material of each of the bodies, and the condition of their surfaces. "Fourthly. On the nature and thickness of the intervening substances, if any. Increase of that thickness diminishes the rate of transfer of heat. "The transfer of heat takes place by three processes, called respectively, radiation, conduction, and convection. "Radiation of heat takes place between bodies at all distances apart, in the same manner and according to the same laws with the radiation of light."* Radiation. To illustrate the radiation of heat from one body to another, take a common poker, heat it to redness in the fire, and hold one hand a few inches from the heated end, as shown in the figure. The hand experiences the sensation called heat, owing to the transfer of the same in straight lines from the hot poker, as it were by radial vibrating rays of heat energy. * From Rankine on The Steam Engine, p. 257. |