112 Prof. Clausius on the Nature of the Motion 
mean distances attract each other, vanish when compared with 
the expansive force due to the motion. But the molecules are 
not always at their mean distances asunder; on the contrary, 
during their motion a molecule is often brought into close proxi- 
mity to another, or to a fixed surface consisting of active mole- 
cules, and in such moments the molecular forces will of course 
commence their activity. The second condition requires, there- 
fore, that those parts of the path described by a molecule under 
the influence of the molecular forces, when the latter are capable 
of altering appreciably the direction or velocity of the molecule’s 
motion, should vanish when compared with those parts of its 
path with respect to which the influence of these forces may be 
regarded as zero. 
If these conditions are not fulfilled, deviations in several ways 
from the simple laws of gases necessarily arise ; and these devia- 
tions become more important the less the molecular condition of 
the gas fulfils the conditions in question. 
On becoming acquainted with the celebrated investigations of 
Regnault on the deviations of gases from Mariotte’s and Gay- 
Lussac’s laws, I attempted, by means of the principles above 
intimated, to deduce some conclusions with respect to the mole- 
cular condition of several gases from the nature of the deviations 
which Regnault detected in the same. <A description of this 
method, however, would be too prolix; and even the results, in 
consequence of the many difficulties encountered in actual calcu- 
lation, are too uncertain to merit being here adduced. 
Whenever, therefore, in the sequel a gas is spoken of, we shall, 
as before, conceive it to be one which perfectly fulfils the above 
conditions, and which Regnault calls an ideal gas, inasmuch as 
all known gases present but an approximation to this condition. 
6. After these considerations on the gaseous condition, the 
question at once arises in what manner the solid and liquid con- 
‘ ditions differ from the gaseous. Although a definition of these 
states of aggregation, in order to be satisfactory in all its details, 
would require a more complete knowledge than we at present 
possess of the condition of the individual molecules, yet it ap- 
pears to me that several fundamental distinctions may be ad- 
vanced with tolerable probability. 
A motion of the molecules takes place in all three states of 
aggregation. 
In the solid state, the motion is such that the molecules move 
about certain positions of equilibrium without ever forsaking the 
same, unless acted upon by foreign forces. In solid bodies, 
therefore, the motion may be characterized as a vibrating one, 
which may, however, be of a very complicated kind. In the first 
place, the constituents of a molecule may vibrate among them- 
