Mechanical Theory of Heat to the Steam Engine. 181 
sorbs only so much heat as ts consumed in doing the external work 
ormed, and that we find in this way many values which, at 
the higher temperatures at least, deviate considerably from the 
laws of Gay Lussac and Mariotte. : 
is view of the behavior of steam was not shared at that 
e even by authors who occupied themselves specially with 
the mechanical theory of heat. W. Th 
tested the point. He found—even a year later in a memoir laid 
to test the correctness of this assumption experimentally. They 
have in fact found by a series of well devised experiments con- 
ducted upon a large scale, that the assumption is so nearly correct 
for the permanent gases examined by them, namely, atmospheric 
air and hydrogen, that the variations may in most calculations be 
neglected. They found, however, greater variations for the non- 
permanent gas, carbonic acid, which they also studied. This 
corresponds entirely with the remark, which I added to the first 
mention of the assumption, that it is noe true for every gas 
Mariotte and Gay 
_ Lussac find their application to the same gas. In consequence of 
these experiments, Foctioon has now also calculated the volume 
of saturated steam in the same way as myself. I believe there- 
fore that the correctness of this mode of calculation will gradu- 
ally be more and more fully recognized by other physicists also, 
3. These two examples will suffice to shew that the fundamen- 
tal principles of the former theory of the steam engine have 
undergone such important changes through the mechanical the- 
ory of heat that a new investigation of the subject is n ; 
e present memoir I have made the attempt to develop the 
tainly well worthy of consideration—to apply steam in an over- 
heated state 
In setting forth this investigation I shall only suppose as known 
my last published memoir* “On an form of the second 
principal theorem of the mechanical theory of heat.” It is true 
that it will in this way be necessary to deduce a second time in a 
somewhat different manner some results which are no longer new, 
but which were obtained at an earlier period by other writers or 
by myself; I believe however, that this repetition will be justi- 
fied by the greater unity and clearness of the whole. 
I shall refer in the proper places to the papers in which these 
tesults were first communicated, as far as they are known to me. 
* Pogg. Ann., xciii, 481. 
