substance/material that is distinctive from matter substances. It became known as
calorific fluid or caloric. The caloric theory considered “heat energy” and “heat
flow” synonymous terms, i.e., caloric. A heat flow process was simply the flow of
the caloric fluid, which was measured by its own unit, the calorie—one calorie is
defined as that amount of caloric fluid when it is absorbed by one gram of water at
constant atmospheric pressure leads to a temperature rise from 14.5 to 15.5 °C.
When caloric flows into a body, it results in a gain in the body of the same caloric.
It is useful to view caloric theory of heat in terms of what is heat, the process of
heat flow, and how is it measured: the materiality of heat hypothesizes what heat is,
the Fourier law and the Clausius Statement (see Sect. 4.1) describe/observe how
heat flows naturally from hot to cold, and calorimetry provides an operational
definition of heat, thus, how heat is measured quantitatively. It turns out that, of the
three ways of understanding heat in accordance with the caloric theory, only the
notion of heat as “material-like” caloric was an illusion and, in any case, a notion
that we can do without, while the latter two are still valid today: we still study how
heat flows down the temperature gradients and we still measure the heat value of
foods in terms of their calorie. This chapter provides a summary of the operational
definition of heat or heating according to the caloric theory of heat.
A rival hypothesis, the mechanical theory of heat (or, the dynamical theory of
heat)—heat is vis viva, the result of the internal random motions of the molecules of
bodies—was less developed than the caloric theory at that time. But, it could also
explain the transmission of heat and the approach to thermal equilibrium between
unequally heated bodies put in contact. Proponents of the caloric theory, such as
Lavoisier and Laplace, considered the dynamical theory as a serious but less successful competitor of the caloric theory. In fact, they took a suspension-of-judgment
position with regards to the ontological nature of heat as shown in this statement
from Memoir on Heat [2].
We shall not decide between these two hypotheses. Several phenomena seem favorable to
the latter [the dynamical theory hypothesis], such as, for example, the heat produced by the
friction of two solid bodies. But there are other phenomena that are explained more simply
by the first hypothesis [the caloric theory]. Perhaps both occur simultaneously. Whatever
may be the case, since only these two hypotheses about the nature of heat can be devised,
we should accept the principles common to both…in a simple mixture of bodies the
quantity of free heat remains always the same. This is evident, if heat is a fluid that tends to
reach equilibrium and also if it is only the vis viva which results from the internal motion of
matter, where the principle in question follows from the conservation of vires vivae. The
conservation of free heat, in the simple mixture of bodies, is thus independent of any
hypothesis about the nature of heat.
It is certainly true that, in the simple problems of thermal mixing of bodies, heat
is conserved since the totality of all interacting bodies is a thermally isolated system
(see Sect. 2.2). For such problems, the conservation of heat is simply a special case
of the conservation of energy.
The more intriguing point is that caloricists such as Lavoisier, Laplace, Poisson,
and Clapeyron were able to advance the science of heat by making use of the
caloric theory formalism to problems that are not thermally isolated, either locally
26
2 Calorimetry and the Caloric Theory of Heat …
calorific fluid or caloric. The caloric theory considered “heat energy” and “heat
flow” synonymous terms, i.e., caloric. A heat flow process was simply the flow of
the caloric fluid, which was measured by its own unit, the calorie—one calorie is
defined as that amount of caloric fluid when it is absorbed by one gram of water at
constant atmospheric pressure leads to a temperature rise from 14.5 to 15.5 °C.
When caloric flows into a body, it results in a gain in the body of the same caloric.
It is useful to view caloric theory of heat in terms of what is heat, the process of
heat flow, and how is it measured: the materiality of heat hypothesizes what heat is,
the Fourier law and the Clausius Statement (see Sect. 4.1) describe/observe how
heat flows naturally from hot to cold, and calorimetry provides an operational
definition of heat, thus, how heat is measured quantitatively. It turns out that, of the
three ways of understanding heat in accordance with the caloric theory, only the
notion of heat as “material-like” caloric was an illusion and, in any case, a notion
that we can do without, while the latter two are still valid today: we still study how
heat flows down the temperature gradients and we still measure the heat value of
foods in terms of their calorie. This chapter provides a summary of the operational
definition of heat or heating according to the caloric theory of heat.
A rival hypothesis, the mechanical theory of heat (or, the dynamical theory of
heat)—heat is vis viva, the result of the internal random motions of the molecules of
bodies—was less developed than the caloric theory at that time. But, it could also
explain the transmission of heat and the approach to thermal equilibrium between
unequally heated bodies put in contact. Proponents of the caloric theory, such as
Lavoisier and Laplace, considered the dynamical theory as a serious but less successful competitor of the caloric theory. In fact, they took a suspension-of-judgment
position with regards to the ontological nature of heat as shown in this statement
from Memoir on Heat [2].
We shall not decide between these two hypotheses. Several phenomena seem favorable to
the latter [the dynamical theory hypothesis], such as, for example, the heat produced by the
friction of two solid bodies. But there are other phenomena that are explained more simply
by the first hypothesis [the caloric theory]. Perhaps both occur simultaneously. Whatever
may be the case, since only these two hypotheses about the nature of heat can be devised,
we should accept the principles common to both…in a simple mixture of bodies the
quantity of free heat remains always the same. This is evident, if heat is a fluid that tends to
reach equilibrium and also if it is only the vis viva which results from the internal motion of
matter, where the principle in question follows from the conservation of vires vivae. The
conservation of free heat, in the simple mixture of bodies, is thus independent of any
hypothesis about the nature of heat.
It is certainly true that, in the simple problems of thermal mixing of bodies, heat
is conserved since the totality of all interacting bodies is a thermally isolated system
(see Sect. 2.2). For such problems, the conservation of heat is simply a special case
of the conservation of energy.
The more intriguing point is that caloricists such as Lavoisier, Laplace, Poisson,
and Clapeyron were able to advance the science of heat by making use of the
caloric theory formalism to problems that are not thermally isolated, either locally
26
2 Calorimetry and the Caloric Theory of Heat …