Another unit of heat that suggests itself quite naturally is the quantity of heat given out by a pound of steam when it condenses into a pound of water at the same temperature. A calorimeter based upon this idea was also used by Bunsen, but the steam calorimeter was i brought to present excellent form largely through the labors of Dr. J. Joly. In his type of the instrument the object to be studied is suspended from one arm of a delicate balance. After being accurately counterpoised, the object is bathed in an atmosphere of steam, with the result that it absorbs a certain amount of heat as its temperature rises to that of the steam. But the heat thus absorbed by the body under examination can be obtained only from the steam itself ; and, since saturated steam cannot part with heat in this way without condensing, it follows that there is deposited upon the body a weight of condensed moisture that corresponds precisely to the quantity of heat that has been absorbed. The amount of this moisture is de termined by careful weighing; and it is evident that the quantity of heat absorbed by the ex perimental body in passing from its original temperature to the temperature of the steam is then immediately known, if we take, as the unit of heat, the quantity of heat that is given out by a pound of steam in condensing into a pound of water at the same temperature. In practice, numerous corrections are of course necessary, as with all other instruments of pre cision. It may be added that although the ice and the steam calorimeters are primarily in tended to determine the heat emitted or ab sorbed by a body in passing from any given tem perature to some one particular temperature that is always the same (that is, the freezing point. in the one case and the boiling-point in the other), yet it is always possible to deter mine the quantity of heat emitted or absorbed by the body between any two temperatures, by performing two experiments in succession, the body having these respective temperatures as its initial temperatures in the respective experi ments. It is plain that the quantity of heat emitted or absorbed between the proposed in itial and terminal temperatures can then be obtained by simply subtracting one of these results from the other.
Another and more familiar unit of heat is the quantity of heat required to warm a given weight of water one degree on a given ther mometric scale. (See CALotuc). Thus in gen eral engineering practice in the United States i and in England, it is customary to define a heat unit as the quantity of heat that is required in order to raise the temperature of a pound of water one degree on the Fahrenheit scale. This definition is good enough for rough purposes, because it conveniently happens that there is no great difference between the quantity of heat required to warm a pound of water from 32° to 33° and the quantity required (for example) to warm it from 99° to 100°. This, however, we can only regard as a fortunate accident; and for accurate scientific purposes we must recog nize that the equality is only approximate, and we must adopt some particular temperature range as a part of our definition. Thus it is common to define the British heat unit, when great accuracy is desired, as the quantity of heat required to raise the temperature of a pound of water from 59° to 60'; although some authorities, apparently without sufficient reason, make the temperature range from 32° to and others have chosen other positions on the temperature scale for the defining degree. It is unfortunate that no general agreement has yet been reached on this point. In accurate sci entific work the unit of heat is usually taken as . _ the quantity of heat required to warm a kilo gram of water from 15" C. to 16° C., or (which is practically the same thing) from 14.5° to 15.5° C. It would appear that several very good reasons could be assigned for selecting 40° C. as the standard temperature to be used in de fining the heat unit. For example, the specific heat of water has its minimum value not far from that point; or, in other words, any small uncertainty in the actual realization of the tem perature contained in the definition would have little or no effect if that temperature were C.
Again C. is the temperature at or near which the differences between the various thermometer scales that are in practical use reach their maximum; and this means that at or near this temperature a slight error in the standardi zation of the thermometer that is used would have the least effect upon the verification of the heat unit. Moreover, 40° C. (104° F.) is a temperature that is likely to be always greater than the general temperature of the laboratory in which work is being carried out; and it is well known to be easier to realize a tempera ture that is higher than that of the surrounding air, than it is to realize one that is lower. From every point of view, therefore, 40° C. (or there abouts) would appear to be the best temperature to assume in establishing the definition of the heat unit; a unit of heat being then defined as the quantity of heat required to raise the tem perature of a kilogram of water from (say) 39° C. to 40° C. Yet, cogent as these reasons would appear, no authority has yet suggested this particular temperature as the standard.
In measuring the quantity of heat emitted by a body by observing the change of tempera ture produced in a given mass of water when the water absorbs the heat so emitted, a great variety of forms of apparatus may be used. In some cases the heated body may be plunged into the water directly, the water being kept well stirred, and its temperature taken at the be ginning and end of the experiment. In other cases, and especially when the body under ex amination cannot be allowed to come in con tact with the water, it is necessary to adopt some more elaborate method, such as enclosing the experimental body in a water-tight envelope of some kind, and afterward making due allow ance for the heat capacity of the envelope. In cases, for example, in which the heat generated by the combustion of fuel is to be measured, the fuel must be enclosed in an air-tight cruci ble, to which oxygen is admitted by one tube, and from which the products of combustion are drawn off by another. The crucible is sur rounded by a mass of water that is disposed in such a way as to intercept and absorb as much of the heat that is produced as possible. A direct observation of the temperature of the water in the calorimeter is made before and after the combustion, and the change of tem perature so obtained gives a first approximation to the amount of heat that has been liberated.
This result has to be corrected, however, for the thermal capacity of each part of the calori meter that has been warmed during the experi ment, and for that of the gases admitted and drawn off, and also for any loss of heat that may have occurred through radiation. The pre cise details of the corrections will vary, how ever, with the design of the calorimeter, and with the mode of conducting the experiments.
For a discussion of the relations of the dif ferent units of heat that have been mentioned above, and for an account of the experiments that have been made for determining the dif ferences in the heat capacities of water at different temperatures, see HEAT. Calorimeters (q.v.) constructed on a large scale are used to measure the amount of heat given off by an animal or human being, the amount of food and air supplied being recorded. Considerable success has been attained in ascertaining the fuel value of various foods by W. O. Atwater and by the Nutrition Laboratory of the Car negie Institution of Washington. A very good account of the subject of calorimetry eral will be found in Preston's Theory of Heat> (London 1894), which also contains val uable references to original papers. The vari ous forms of calorimeter that are used in prac tical engineering are explained and illustrated in Carpenter's (Text-Book of Experimental En gineering' and in almost all general textbooks on physics. Consult Transactions of the Royal Society of London> (1894), and Wiedemann's der Physik and der Chemie> (Vol. XXXVII, p. 494, 1889). See FUEL.