CAPACITY, in the modern doctrine of heat, signifies the proportional capabi lity of a given quantity of any substance to absorb and retain caloric, or that dis position or property, by which various bodies respectively require more or less of this fluid to superinduce any given temperature in a given mass. See CA LORIC. • That this capacity varies in different bodies, and even in the same substance in different states. may be easily shewn. If the quantities of heat necessary to be added to or taken from bodies, in order to produce equal changes in their tem perature, were in all cases proportional to their respective quantities of matter ; as if, for example, it would require the same quantity of this fluid to heat a pound of water, a pound of oil, or a pound ofmercury, 20 degrees, this would, of course, indicate that their capacities were equal : but if, on the contrary, it should be found that the same quantity of caloric, applied to these various sub stances, should produce different changes in the temperature of equal quantities, or equal changes in the temperature of different quantities of each, it would fol low, that their capacities from this fluid must proportionally vary. Let us con ceive, that having three several pounds of water at the temperature of 110° of Fahrenheit's thermometer in separate vessels, there be added to the first a quantity of water at 50° ; to the second a quantity of spermaceti oil, also at 50° ; and to the third a quantity of mercury at the like temperature of 50° ; and that each of the mixtures be stirred together, and the addition continued, till they have all assumed throughout a common tem perature of Now, as each of the pounds of water has, in this case, been deprived of an equal quantity of caloric, (viz. as much as was necessary to raise its temperature 40°, or from 70° to 110°, the absolute capacities of the whole of the water, the oil, and the mercury, which have been added, must, of course, be equal, whatever be the quantity of each ; each of them having absorbed an equal quantity of heat. On comparing the quantities of these latter substances, however, it will be found that we have employed in the experiment about two pounds of water at 50°, fourpounds of oil, and nearly sixty pounds of mercury, each of which has been heated 20° ; so that it requires as much caloric to heat one pound of water 20°, as to pftoduce the same effect on two of oil, or 30 of mercury ; and their relative capacities are therefore inversely in this proportion.
A change of capacity in the same body is producible in three ways : by mecha nical compression or dilatation; by che mical combination, or by the action of heat itself, of each of which we shall say a few words. With regard to the first, the general fact appears to be, that wherever a body is by any means con densed, its capacity becomes diminished ; but that where it is dilated or enlarged in its bulk, it is proportionally increased. Thus, if a thermometer be suspended in a receiver, and a quan ity of air con densed into it, the mercury will rise ; a part of the caloric which is contained in the air being, as it were, squeezed out by its compression, and forced into the mercury in the bulb, whose tempera, ture is consequently raised : if; however, on the contrary, the air be rarefied, the thermometer will indicate cold ; the ca pacity of the air in the receiver being increased by its rarefaction, and a por tion of the caloric in the contiguous bo dies consequently absorbed, whereby their temperature is lowered and their bulk diminished.
The second mode of changing the ca pacities of bodies is by their chemical combination ; and, perhaps, there is no combination unaccompanied by such a change. In some instances this takes place in a very remarkable degree, and it is from hence that we derive the effects of calorific and frigorific mixtures. if, for example, a quantity of sulphuric acid, diluted with an equal measure of water, be poured on a quantity of crystals of Glauber% salt, recently powdered, the capacity of the compound is considerably greater than that of its component ingre dients; it becomes, therefore, strongly absorbent of caloric, which it attracts from the bodies in its vicinity, and a quan tity of water in a phial placed in the mix ture will be soon frozen.