and the Constitution of Elastic Fluids. 213 
air was found to be the equivalent of the mechanical power em- 
ployed, and, vice versd, the heat absorbed in rarefaction was 
found to be the equivalent of the mechanical power developed, 
estimated by the weight of the column of atmospheric air dis- 
placed. In the case of compressed air expanding into a vacuum, 
since no mechanical power was produced, no absorption of heat 
was expected or found. M. Seguin has confirmed the above 
results in the case of steam. 
The above principles lead, indeed, to a more intimate acquaint- 
ance with the true theory of the steam-engine; for they have 
enabled us to estimate the calorific effect of the friction of the 
steam in passing through the various valves and pipes, as well as 
that of the piston in rubbing against the sides of the cylinder ; 
and they have also informed us that the steam, while expanding 
in the cylinder, loses heat in quantity exactly proportional to the 
mechanical force developed*. 
The experiments on the changes of temperature produced by 
the rarefaction and condensation of air give likewise an insight 
into the constitution of elastic fluids, for they show that the heat 
of elastic fluids is the mechanical force possessed by them; and 
since it is known that the temperature of a gas determines its 
elastic force, it follows that the elastic force, or pressure, must 
be the effect of the motion of the constituent particles in any 
gas. This motion may exist in several ways, and still account 
for the phenomena presented by elastic fluids. Davy, to whom 
belongs the signal merit of having made the first experiment 
absolutely demonstrative of the immateriality of heat, enunciated 
the beautiful hypothesis of a rotatory motion. He says, “It 
seems possible to account for all the phenomena of heat, if it be 
supposed that in solids the particles are in a constant state of vibra- 
tory motion, the particles of the hottest bodies moving with the 
greatest velocity and through the greatest space: that in fluids 
and elastic fluids, besides the vibratory motion, which must be 
considered greatest in the last, the particles have a motion round 
their own axes with different velocities, the particles of elastic 
fluids moving with the greatest quickness; and that in etherial 
substances the particles move round their own axes, and separate 
from each other, penetrating in right lines through space. ‘Tem- 
perature may be conceived to depend upon the velocity of the 
vibrations, increase of capacity on the motion being performed 
* A complete theory of the motive power of heat has been recently com- 
municated by Professor Thomson to the Royal Society of Edinburgh. In 
this paper the very important law is established, that the fraction of heat 
converted into power in any perfect engine, is equal to the range of tempe- 
on hens by the highest temperature above absolute zero.— May 185], 
