CAPACITY. 
for wine, beer, com, salt, &.'C. are called 
jneasiires of capacity. 
Capacity, in the modern doctrine of 
heat, signifies the proportional capability of 
a given quantity of any substance to ab- 
sorb and retain caloric, or that disposi- 
tion or property by which various bodies 
respectively require more or less of this 
fluid to superinduce any given temperature 
in a riven mass, See Caloric. 
That this capacity varies in different bo- 
dies, 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 pro- 
duce equal changes in their temperature, 
were in all cases proportional to their re- 
spective quantities of matter ; — as if, for 
example, it would require the same quan- 
tity of this fluid to heat a pound of water, 
a, pound of oil, ora pound of mercury 20 
degrees ; this would, of course, indicate 
tliat their capacities were equal ; but if, 
on the contrary, it should be found that the 
same quantity of caloric applied to these 
various substances, should produce different 
changes in the temperature of equal quanti- 
ties, or equal changes in the temperature of 
different quantities of each>it would follow, 
that their capacities for this fluid must pro- 
portionally vaiy. Let us conceive, that hav- 
ing three several pounds of water at the 
temperature of 110" of Fahrenheit’s ther- 
mometer 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" j and to the third a quantity of 
mercury at the like t(;inperature of 50° ; 
and that each of the mixtures be stirred to- 
gether, and the addition continued, till 
they, have all assumed throughout a com- 
mon temperature of 70". Now, as each 
of the pounds of water has, in this case, 
been deprived of an equal quantity of ca- 
loric, (r’izi as much as was necessary to 
raise its temperature -SO", 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 quan- 
tities of these latter substances, however, 
it will be found that we have employed in 
the experim.ent about 2 pounds of water 
at 50°, 4 pounds of oil, and nearly 60 pounds 
of mercuiy , each of which has been lieated 
20° ; so that it requires as much caloric to 
heat one pound of water 20", as to produce 
the same etfect on two of oil, or .“50 ofni€t'» 
cury ; and tlieir relative capacities are 
therefore inversely in this proportion. A 
change of capacity in the same body is 
producible in three ways: by mechani- 
cal compression or dilatation, by chemical 
combination, or by the action of heat it- 
self, of each of which we shall say a few 
words. With regard to the first, the gene- 
ral fact appears to be, that wherever a body 
is by any means condensed, its capacity be- 
comes diminished, but that where it is di- 
lated or enlarged in its bulk, it is propor- 
tionally increased. Thus, if a tliermometer 
be suspended in a receiver, and a quantity 
of air condensed into it, the mercury will 
rise ; a part of the caloric which is con- 
tained in the air being, as it were, squeezed 
out by its compression, and forced into the 
mercury in the bulb, whose temperature is 
consequently raised : if, however, on the 
contiaiy, the air be rarified, the thennome- 
ter will indicate cold ; the capacity of the 
air in the receiver being increased by its 
rarefaction, and a portion of the caloric in 
the contiguous bodies consequently absorb- 
ed, whereby tiieir temperature is lowered 
and their bulk diminished. 
The second mode of clianging the capa- 
cities of bodies is by their chemical combi- 
nation ; and, perhaps, tliere is no combina- 
tion unaccompanied by such a change. In 
some instances this takes place in a very re- 
markable degree, and it is from hence that 
we derive the effects of calorific and frigo- 
rific mixtures. If, for example, a quantity 
of sulphuric acid, diluted with an equal 
measure of water, be poured on a quantity 
of crystals 6f Glauber’s salt, recently pow- 
dered, the capacity of the compound is con- 
siderably greater than that of its compo- 
nent ingredients; it becomes, therefore, 
strongly absorbent of caloric, which it at- 
tracts from the bodies in its vicinity, ami a 
qtiantity of water in a phial placed in the 
mixture will be soon frozen. 
The third case of change of capacity, by 
the action of heat itself, is, perhaps, pro- 
ductive of more important effects in nature 
than either of the other two. The capaci- 
ties of all bodies are increased in some pro- 
portion to the dilatation of their bulk, and 
the disaggregation of their eonstituent par- 
ticles, 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 pkice, heat 
