which we call Heat. 109 
few principal points, and reserve a more complete analysis for 
another time*. 
2. Kronig assumes that the molecules of gas do not oscillate 
about definite positions of equilibrium, but that they move with 
constant velocity in right lines until they strike against other 
molecules, or against some surface which is to them impermeable. 
I share this view completely, and I also believe that the expan- 
sive force of the gas arises from this motion. On the other 
hand, I am of opinion that this is not the only motion present. 
In the first place, the hypothesis of a rotatory as well as a 
progressive motion of the molecules at once suggests itself; for 
at every impact of two bodies, unless the same happens to be 
central and rectilineal, a rotatory as well as a translatory motion 
ensues. 
I am also of opinion that vibrations take place within the 
several masses in a state of progressive motion. Such vibrations 
are conceivable in several ways. Even if we limit ourselves to 
the consideration of the atomic masses solely, and regard these 
as absolutely rigid, it is still possible that a molecule, which con- 
sists of several atoms, may not also constitute an absolutely rigid 
mass, but that within it the several atoms are to a certain extent 
moveable, and thus capable of oscillating with respect to each 
other. 
I may also remark, that by thus ascribing a movement to the 
atomic masses themselves, we do not exclude the hypothesis that 
each atomic mass may be provided with a quantity of finer mat- 
ter, which, without separating from the atom, may still be move- 
able in its vicinity. 
By means of a mathematical investigation given at the end of 
the present memoir, it may be proved that the vis viva of the 
translatory motion alone is too small to represent the whole heat 
present in the gas; so that without entering into the probability 
of the same, we are thus compelled to assume one or more mo- 
tions of another kind. According to this calculation, the excess 
of the whole vis viva over that of the translatory motion alone is 
particularly important in gases of a complicated chemical consti- 
tution, in which each molecule consists of a great number of atoms. 
* T must not omit to mention here, that some time ago Mr. William 
Siemens of London, when on a visit in Berlin, informed me that Joule 
had also expresscd similar ideas in the Memoirs of the Literary and Philo- 
sophical Society of Manchester. My views being consequently no longer 
completely new, this was an additional reason why I should hasten their 
genes less than I otherwise probably should have done. Hitherto I 
ve not been able to procure the memoir of Joule in question, and there- 
fore I am ignorant how far he has pursued the subject, and whether his 
views coincide with mine in all points. It is to be regretted that Joule did 
not publish his memoir in a more widely circulated periodical, 
