280 
clearly as possible before his mind the main results arrived at, 
and the general features of the apparatus employed. 
In the above diagram, we have a graphical representation of 
the results cf a large number of comparative experiments on air 
and carbonic acid, under pressures ranging from 48 to 107 atmo- 
spheres, and at temperatures for the carbonic acid varying from 
13°1 C, to 48°" C. The dotted lines (zr Curves) represent a 
portion of the curves of a perfect gas (assumed to have the same 
volume originally at 0° C,, and under one atmosphere as the car- 
bonic’acid), for the temperatures of 13°°1 C., 31°°r C., and 48°'1 C. 
The lines designated Carbonic Acid Curves show the volumes to 
which the carbonic acid is reduced at the temperatures marked 
on each curve, and under the approximate pressures indicated by 
the numbers at the top and bottom of the figure. Ordinates 
drawn from the inner horizontal line at the lower part of the figure 
to meet the curves, will represent the volume of the carbonic 
acid. These ordinates do not always refer to homogeneous 
matter, but sometimes to a mixture of gas and liquid. 
It will be observed that in the curves for 13°'r C. there occurs an 
abrupt, or almost quite abrupt, fall, when a pressure of about 49 
atmospheres has been attained. The curve for 21°°5 C. exhibits a 
corresponding fall, but not till a higher pressure (about 60 atmo- 
spheres) has been reached. On close inspection of the figure, a 
slight deviation from perfect abruptness will be observed in the 
portion of the curves representing these falls, which Dr. Andrews 
showed to be due to a trace of air (about =, part) in the carbonic 
acid with which the experiments were made. Had the carbonic 
acid been absolutely pure, there can be no doubt that the fall 
would have been quite abrupt. 
In the curve for 31°'1 C. there isno abrupt fall ; buta rapid de- 
scent, indicating a corresponding diminution of volume, occurs 
between the pressures of 73 and 75 atmospheres. As the tem- 
perature rises this descent becomes gradually less marked, and 
when a temperature of 48°'r C. has been attained, it has almost, if 
not altogether, disappeared. 
At any temperature between — 57°C., and 30°92 C., carbonic 
acid, under the ordinary pressure of the atmosphere, is un- 
questionably in the state of a gas or vapour. If within these 
limits we take a given volume of carbonic acid, and gradually 
augment the pressure, the volume will steadily diminish, not 
however uniformly, but according to a more rapid rate than the 
law for a perfect gas, till we reach the point at which liquefaction 
begins. A sudden fall or diminution of volume will now take 
place, and with a little care it will be found easy so to arrange 
the experiment that part of the carbonic acid shall be in the 
liquid, and part of it in the gaseous state ; the carbonic acid 
thus coexisting in two distinct physical conditions in the same 
tube, and under the same external pressure. But if the experi- 
ment be made at 30°92 C., or any higher temperature, the re- 
sult will be very different. At 30°°92 C., and under a pressure 
of about 74 atmospheres, the densities of liquid and gaseous car- 
bonic acid, as well as all their other physical properties, become 
absolutely identical, and the most careful observation fails to 
discover any heterogeneity at this or higher temperatures in 
carbonic acid, when its volume is so reduced as to occupy a space 
in which, at lower temperatures, a mixture of gas and liquid 
would have been formed. In other words, all distinctions 
of state have disappeared, and the carbonic acid has become 
one homogeneous fluid, which cannot by change of pressure be 
separated into two distinct physical conditions. This tem- 
perature of 30°°92 is called by Dr. Andrews the critical point 
of carbonic acid. Other fluids which can be obtained in both 
the liquid and gaseous states have shown similar phenomena, 
and have each presented a critical point of temperature. The 
rapid changes of density which slight changes of temperature 
or pressure produce, when the gas is reduced at temperatures a 
little above the critical point, to the volume at which it might 
be expected to liquefy, account for the flickering movements 
referred to in the beginning of this article. 
The general conclusions arrived at we give in the words of the 
original memoir. ‘‘T have frequently exposed carbonic acid,” 
observes Dr. Andrews, ‘* without making precise measurements, 
to much higher pressures than any marked in the tables, and 
have made it pass, without break or interruption, from what is 
regarded by every one as the gaseous state, to what is, in like 
manner, universally regarded as the liquid state. Take, for ex- 
ample, a given volume of carbonic acid gas at 50° C., or ata 
higher temperature, and expose it to increasing pressure till 150 
atmospheres have been reached. In this process its volume will 
steadily diminish as the pressure augments, and no sudden 
NATURE 
diminution of volume, without the application of external 
pressure, will occur at any stage of it. When the full 
pressure has been applied, let the temperature be allowed to 
fall, till the carbonic 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 
series of gradual changes, presenting nowhere any abrupt altera- 
tion of volume or sudden evolution of heat, it ends with a liquid. 
The closest observation fails to discover anywhere 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 suspected, 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 car- 
bonic acid, and its subsequent cooling; but these operations 
might have been performed simultaneously, if care were taken 
so to arrange the application of the pressure and the rate of cool- 
ing 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 yolume of the liquid, without giving evidence at any part 
of the process of liquefaction having occurred? Does it con- 
tinue in the gaseous state, or does it liquefy, or have we to deal 
with a new condition of matter? If the experiment 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 con- 
dition during the compression ; and few would hesitate to de- 
clare 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 experiment 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 discover 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 ap- 
pears to me to be the true interpretation of the experiments 
already described, is to be found in the close and intimate re- 
lations 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 true, 
in what may be described as a state of instability, and sud- 
denly passes, with the evolution of heat, and without the appli- 
cation of additional pressure or change of temperature, 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 difference is exhibited, while 
the process is going on, in the optical and other physical 
properties 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 <4; 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 positive reply. Carbonic acid at 35°5, and under ro8 atmo- 
{ 
[Aug. 4, 1870 
