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The nucleus of truth in the doctrine, around which lias 
gathered no slight amount of ambiguity and pretentious false- 
hood, consists of three main elements. 
First, a separate fluid of Heat or Caloric, the usual theory of 
last century, and the basis of the treatises of Fourier and 
Poisson, has been set aside. The earlier view of Lord Bacon, 
that heat is a special form of atomic motion, held since by Locke, 
Rumford, and many others, has gained a complete triumph. 
By the skilful researches of Joule, Seguier, Thomson, and others, 
the number of feet of elevation, which answer in mechanical force 
to a degree of temperature, has been very nearly determined. 
In an age of steamboats and railroads, such a determination is 
of value to engineers, and is well adapted to arrest the popular 
mind, and seal the triumph of the corrected theory. But in 
point of abstract science, it is a detail of slight importance. 
Some such equivalence is a self-evident result, when the view 
of heat as atomic motion has once been received. 
The second truth is wider and more comprehensive. The 
walls, which parted asunder different classes of motion, or 
modes of atomic force, have been slowly removed. Hypothesis 
took the form, in the last century, of inventing distinct fluids 
for each main set of phenomena to be explained. Thus, in 
different works, we had one or two kinds of electric fluid, one 
or two of magnetic, a separate fluid of heat or caloric, and a 
luminiferous ether, or else a substance of light, shot out with 
immense velocity. But the progress of research has broken 
down these artificial barriers. Electro-galvanism, electro-mag- 
netism, thermo-electricity, thermo-magnetism, actinism, and the 
polarization of heat and light, have bridged over the limits of 
separation. A heptarchy of sciences has been changed into an 
united monarchy. All these phenomena are now referred to 
one ethereal medium, in conjunction with ponderable matter; 
while some hold that even tins is not required, and refer all 
these changes to the affections of matter alone. 
The third element is more important. Let us assume the 
only forces of a system to be of the same class with gravitation, 
— attractions or repulsions, that depend only on the distance 
of the atoms, and increase by some definite law when the distance 
is lessened. A simple relation between the initial and final 
distances, and the motions produced, will then result from 
pure dynamical reasoning. However complex the system and 
its motions, the amount of motion generated or destroyed will 
not depend on the paths of the particles, but on the first and 
last distances alone. This truth, under the old name, Conser- 
vation of Vis viva , has been familiar to mathematicians ever 
