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sumed, the rest of which (often the greater part), insensible to the ther-
mometer, is absorbed to modify the physical constitution.
This is what we mean when we say that a portion of heat has be-
come latent; a term which we may retain, though it was originally used
in connection with a theory about the nature of heat. This is the funda-
mental law discovered by Black, by observation of the indisputable cases
in which a physical modification takes place without any change of
temperature in the modified body. When the two effects co-exist, it is
much more difficult to analyse and apportion them.
Considering first the laws of change of it is a general truth that
every homogeneous body dilates with heat and contracts with cold; and
the fact holds good with heterogeneous bodies, such as organized tis-
sues especially, in regard to their constituent parts. There are very few
exceptions to this rule, and those few extend over a very small portion
of the thermometrical scale. The principal anomaly however being the
case of water, it has great importance in natural history, though not
much in abstract physics, except from the use that philosophers have
made of it to procure an invariable unity of density, always at com-
mand. These anomalies, too rare and restricted to invalidate any general
law, are sufficient to discredit all a priori explanations of expansions
and contractions, according to which every increase of temperature should
cause an expansion, and every diminution a contraction, contrary to the
facts.
Solids dilate less than liquids, under the same elevation of tempera-
ture, and liquids than gases; and not only when the same substance
passes through the three states, but also when different substances are
employed. The expansion of solids proceeds, as far as we know, with
perfect uniformity. We know more of the case of liquids, which is ren-
dered extremely important from its connection with the true theory of
Positive Philosophy/245
the thermometer, without which all thermological inquiries would be
left in a very dubious state. Experiments, devised by Dulong and Petit,
have shown that for above three hundred centigrade degrees the expan-
sion of the mercury follows an exactly uniform course, equal increase
of volume being produced by heat able to melt equal weights of ice at
zero. This is the only case fully established; but we have reason to be-
lieve that the rule extends to that of all liquids. The most marked case of
such regularity is that of gases. Not only does the expansion take place
by equal gradations, as usually in liquids and solids, but it affects all
gases alike. Gases differ from each other, like liquids and solids, in their
density, their specific heat, and their permeability; vet they all dilate
uniformly and equally, their volume increasing three-eighths, from the
temperature of melting ice to that of boiling water. Vapours are like
gases in this particular, as in so many others. These are the simple gen-
eral laws of the expansion of elastic fluids, discovered at once by Gay-
Lussac at Paris, and Dalton at Manchester.
Next, we have to notice the changes produced by heat in the state of
aggregation of bodies.
Solidity and fluidity used to be regarded as absolute qualities of
bodies; whereas, we now know them to be relative, and are even certain
that all solid bodies might be rendered fluid if we could apply heat enough,
avoiding chemical alteration. In the converse way, we used to suppose
that gases must preserve their elasticity, through all degrees of cooling
and of compression; whereas Bussy and Faraday have shown us that
most of them easily become liquid, when they are seized in their nascent
state; and there is every reason to believe that by a due combination of
cold and pressure, they may be always liquefied, even in their developed
state. Under this view therefore, different substances are distinguished
only by the different parts of the indefinite thermometrical scale to which
their successive states, solid, liquid, and gaseous, correspond. But this
simple inequality is an all-important characteristic, which is not yet thor-
oughly connected with any other fundamental quality of each substance.
Density is the relation which is the least obscure and variable, gases
being in general less dense than liquids, and liquids than solids. But
there are striking exceptions in the second case, and might be in the
first, if we knew more of gases in regard to compression, and in varied
circumstances of other kinds. As for the three states of the same sub-
stance, there is always, except in some cases of scarce anomaly, rar-
efaction in the fusion of solids and in the evaporation of liquids. All
246/Auguste Comte
these changes have been brought by the illustrious Black under one fun-
damental law, which is both from its importance and its universality,
one of finest discoveries in natural philosophy. It is this: that in the
passage from the solid to the liquid state, and from that to the gaseous,
every substance always absorbs a more or less distinguishable quantity
of heat, without raising its temperature; while the inverse process occa-
sions a disengagement of heat, precisely correspondent to the absorp-
tion. These disengagements and adsorptions of heat are evidently, after
chemical phenomena, the principal sources of heat and cold. In an ex-
periment of Leslie s, an evaporation, rendered extremely rapid by artifi-
cial means, has produced the lowest temperatures known. Eminent natural
philosophers have even believed that the heat which is so abundantly
disengaged in most great chemical combinations could proceed only
from the different changes of state which commonly result from them.
But this opinion, though true in regard to a great number of cases has
too many exceptions to deal with to become a general principle.
We have now done with physical thermology. But the laws of the
formation and tension of vapours now form an appendix to it; and also
of course hydrometry. The theory is, in fact, the necessary complement
of the doctrine of changes of state; and this is its proper place.
Before Saussure s time, evaporation was regarded as chemical fact,
occasioned by the dissolving action of air upon liquids. He showed that
the action of the air was adverse to evaporation, except in the case of the
renovation of the atmosphere. The test was found in the formation of
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