separated forms of the same condition of matter, and may be made to pass one into the other without any interruption or breach of continuity'.

The diagram, Fig. 1, for carbonic acid is taken from Dr Andrews' paper, with the exception of the dotted line shewing the region within which the substance can exist as a liquid in presence of its vapour. The base line of the diagram corresponds, not to zero pressure, but to a pressure of 47 atmospheres.

The lowest of the isothermal lines is that of 13°1 C. or 55°.6 F.

This line shews that at a pressure of about 47 atmospheres condensation occurs. The substance is seen to become separated into two distinct portions, the upper portion being in the state of vapour or gas, and the lower in the state of liquid. The upper surface of the liquid can be distinctly seen, and where this surface is close to the sides of the glass containing the substance it is seen to be curved, as the surface of water is in small tubes.

As the volume is diminished, more of the substance is liquefied, till at last the whole is compressed into the liquid form.

Liquid carbonic acid, as was first observed by Thilorier, dilates as the temperature rises to a greater degree than even a gas, and, as Dr Andrews has shewn, it yields to pressure much more than any ordinary liquid. From Dr Andrews' experiments it also appears that its compressibility diminishes as the pressure increases. These results are apparent even in the diagram. It is, therefore, far more compressible than any ordinary liquid, and it appears from the experiments of Andrews that its compressibility diminishes as the volume is reduced.

It appears, therefore, that the behaviour of liquid carbonic acid under the action of heat and pressure is very different from that of ordinary liquids, and in some respects approaches to that of a gas.

The

If we examine the next of the isothermals of the diagram, that for 215 C. or 70°-7 F., the approximation between the liquid and the gaseous states is still more apparent. Here condensation takes place at about 60 atmospheres of pressure, and the liquid occupies nearly a third of the volume of the gas. exceedingly dense gas is approaching in its properties to the exceedingly light liquid. Still there is a distinct separation between the gaseous and liquid states, though we are approaching the critical temperature. This critical temperature has been determined by Dr Andrews to be 30°-92 C. or 87°.7 F. At this temperature, and at a pressure of from 73 to 75 atmospheres, 1 Phil. Trans. 1869, p. 575.

No sepa

carbonic acid appears to be in the critical condition. ration into liquid and vapour can be detected, but at the same time very small variations of pressure or of temperature produce such great variations of density that flickering movements are observed in the tube "resembling in an exaggerated form the appearances exhibited during the mixture of liquids of different densities, or when columns of heated air ascend through colder strata."

The isothermal line for 31°1 C. or 88° F. passes above this critical point. During the whole compression the substance is never in two distinct conditions in different parts of the tube. When the pressure is less than 73 atmospheres the isothermal line, though greatly flatter than that of a perfect gas, resembles it in general features. From 73 to 75 atmospheres the volume diminishes very rapidly, but by no means suddenly, and above this pressure the volume diminishes more gradually than in the case of a perfect gas, but still more rapidly than in most liquids.

In the isothermals for 32o5 C. or 90°-5 F. and for 35o-5 C. or 95°-9 F. we can still observe a slight increase of compressibility near the same part of the diagram, but in the isothermal line for 48°.1 C. or 118°.6 F. the curve is concave upwards throughout its whole course, and differs from the corresponding isothermal line for a perfect gas only by being somewhat flatter, shewing that for all ordinary pressures the volume is somewhat less than that assigned by Boyle's law.

Still at the temperature of 118° 6 F. carbonic acid has all the properties of a gas, and the effects of heat and pressure on it differ from their effects on a perfect gas only by quantities requiring careful experiments to detect them.

We have no reason to believe that any phenomenon similar to condensation would occur, however great a pressure were applied to carbonic acid at this temperature.

CAMBRIDGE: PRINTED BY C. J. CLAY, M.A., AT THE UNIVERSITY PRESS.

March 1879.

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