CAPACITY. 



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 of mercury, 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 70. 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, four pounds 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 produce 

 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 bady 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 iiy of air con- 

 densed into it, the mercury will rise ; a 

 part of the caloric which is contained 

 in the air bein,;, as it were, squeezed 

 out by its compression, and forced into 

 the mercury in the bulb, wiiose 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 etlects 

 of calorific and fngonfic 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's 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. 



The third case of change of capacity, 

 by the action of heat itself, is, perhaps, 

 productive of more important effects in 

 nature than either of the other two. The 

 capacities ot all bodies are increased in 

 some proportion to the dilatation of their 

 bulk, and the disa ;gregation of their 

 constituent particles, as well by the 

 agency of caloric as by any other 'cause. 

 Hence, when a solid is fused, or a liquid 

 resolved into vapour, cold is produced 

 by the augmentation of its capacity ; and, 

 e converso, when steam is condensed, or 

 congelation takes place, heat is developed 

 by its diminution. Thus, if equal quan- 

 tities of pounded ice and water, each at 

 S2 of Fahrenheit, be exposed to heat in 

 two similar vessels in a water-bath, the 

 water will be heated in 178 before the 

 ice is all dissolved, the water produced 

 from which will, of course, still remain 



