GASEOUS AND LIQUID STATES OF MATTER 
587 
acid has reached the ordinary temperature of the atmosphere. During the whole of 
this operation no breach of continuity has occurred. It begins with a gas, and by a 
series of gradual changes, presenting nowhere any abrupt alteration of volume or sudden 
evolution of heat, it ends with a liquid. The closest observation fails to discover any- 
where indications of a change of condition in the carbonic acid, or evidence, at any 
period of the process, of part of it being in one physical state and part in another. 
That the gas has actually changed into a liquid would, indeed, never have been sus- 
pected, had it not shown itself to be so changed by entering into ebullition on the 
removal of the pressure. For convenience this process has been divided into two stages, 
the compression of the carbonic acid and its subsequent cooling ; but these operations 
might have been performed simultaneously, if care were taken so to arrange the appli- 
cation of the pressure and the rate of cooling, that the pressure should not be less than 
76 atmospheres when the carbonic acid had cooled to 31°. 
We are now prepared for the consideration of the following important question. 
What is the condition of carbonic acid when it passes, at temperatures above 31°, from 
the gaseous state down to the volume of the liquid, without giving evidence at any part 
of the process of liquefaction having occurred % Does it continue in the gaseous state, 
or does it liquefy, or have we to deal with a new condition of matter l If the experi- 
ment were made at 100°, or at a higher temperature, when all indications of a fall had 
disappeared, the probable answer which would be given to this question is that the gas 
preserves its gaseous condition during the compression ; and few would hesitate to 
declare this statement to be true, if the pressure, as in Natterer’s experiments, were 
applied to such gases as hydrogen or nitrogen. On the other hand, when the experi- 
ment is made with carbonic acid at temperatures a little above 31°, the great fall which 
occurs at one period of the process would lead to the conjecture that liquefaction had 
actually taken place, although optical tests carefully applied failed at any time to dis- 
cover the presence of a liquid in contact with a gas. But against this view it may be 
urged with great force, that the fact of additional pressure being always required for a 
further diminution of volume, is opposed to the known laws which hold in the change 
of bodies from the gaseous to the liquid state. Besides, the higher the temperature at 
which the gas is compressed, the less the fall becomes, and at last it disappears. 
The answer to the foregoing question, according to what appears to me to be the true 
interpretation of the experiments already described, is to be found in the close and inti- 
mate relations which subsist between the gaseous and liquid states of matter. The 
ordinary gaseous and ordinary liquid states are, in short, only widely separated forms of 
the same condition of matter, and may be made to pass into one another by a series of 
gradations so gentle that the passage shall nowhere present any interruption or breach 
of continuity. From carbonic acid as a perfect gas to carbonic acid as a perfect liquid, 
the transition we have seen may be accomplished by a continuous process, and the gas 
and liquid are only distant stages of a long series of continuous physical changes. 
Under certain conditions of temperature and pressure, carbonic acid finds itself, it is 
