588 
DE, ANDEEWS ON THE CONTINUITY OE THE 
true, in what may be described as a state of instability, and suddenly passes, with the 
evolution of heat, and without the application of additional pressure or change of tem- 
perature, to the volume, which by the continuous process can only be reached through 
a long and circuitous route. In the abrupt change which here occurs, a marked differ- 
ence is exhibited, while the process is going on, in the optical and other physical pro- 
perties of the carbonic acid which has collapsed into the smaller volume, and of the 
carbonic acid not yet altered. There is no difficulty here, therefore, in distinguishing 
between the liquid and the gas. But in other cases the distinction cannot be made ; 
and under many of the conditions I have described it would be vain to attempt to 
assign carbonic acid to the liquid rather than the gaseous state. Carbonic acid, at the 
temperature of 35° - 5, and under a pressure of 108 atmospheres, is reduced to of 
the volume it occupied under a pressure of one atmosphere ; but if any one ask whether 
it is now in the gaseous or liquid state, the question does not, I believe, admit of a posi- 
tive reply. Carbonic acid at 35 0- 5, and under 108 atmospheres of pressure, stands 
nearly midway between the gas and the liquid ; and we have no valid grounds for 
assigning it to the one form of matter any more than to the other. The same observa- 
tion would apply with even greater force to the state in which carbonic acid exists at 
higher temperatures and under greater pressures than those just mentioned. In the 
original experiment of Cagniard de la Tour, that distinguished physicist inferred that 
the liquid had disappeared, and had changed into a gas. A slight modification of the 
conditions of his experiment would have led him to the opposite conclusion, that what 
had been before a gas was changed into a liquid. These conditions are, in short, the 
intermediate states which matter assumes in passing, without sudden change of volume, 
or abrupt evolution of heat, from the ordinary liquid to the ordinary gaseous state. 
In the foregoing observations I have avoided all reference to the molecular forces 
brought into play in these experiments. The resistance of liquids and gases to external 
pressure tending to produce a diminution of volume, proves the existence of an internal 
force of an expansive or resisting character. On the other hand, the sudden diminution 
of volume, without the application of additional pressure externally, which occurs when 
a gas is compressed, at any temperature below the critical point, to the volume at which 
liquefaction begins, can scarcely be explained without assuming that a molecular force 
of great attractive power comes here into operation, and overcomes the resistance to 
diminution of volume, which commonly requires the application of external force. 
When the passage from the gaseous to the liquid state is effected by the continuous 
process described in the foregoing pages, these molecular forces are so modified as to be 
unable at any stage of the process to overcome alone the resistance of the fluid to change 
of volume. 
The properties described in this communication, as exhibited by carbonic acid, are 
not peculiar to it, but are generally true of all bodies which can be obtained as gases 
and liquids. Nitrous oxide, hydrochloric acid, ammonia, sulphuric ether, and sulphuret 
of carbon, all exhibited, at fixed pressures and temperatures, critical points, and rapid 
