ON LIQUID AND GASEOUS STATES. 151 



liquid. I may perhaps just mention, to give a popular idea 

 of the question I am speaking roughly that if you descend to 

 1200 feet below the level of the sea, and there examine the properties 

 of carbonic acid, it would exist, at the pressure there produced and 

 at the temperature of o C. (the freezing point of water), in the state of 

 a liquid. If you descend 1700 feet, not a great depth in the sea, you 

 would find that this body would remain a liquid at the temperature of 

 15 C., and if you heated it above that point, it would boil ; it would 

 resemble in this respect the chemical compound hydrochloric ether 

 which boils at the ordinary temperature of the atmosphere. If you 

 descend to a depth of about 2500 feet, and there make your obser- 

 vations, you will find that the carbonic acid would cease to show the 

 properties of an ordinary liquid at all temperatures above 31 C. At 

 the temperature of 15 C. and at a depth of 2500 feet, carbonic acid 

 would be a liquid. If you heated it, instead of boiling, it would 

 change in a most remarkable manner, the surface would lose its 

 curvature, the concave surface with which every one is familiar in an 

 ordinary liquid would gradually become flattened and disappear, and 

 at last the liquid would change, not into the gaseous state, but into 

 one of those intermediate conditions which I have described as con- 

 necting the liquid and gaseous states together. At this depth of 2500 

 feet and at 31 C., which I have designated the "critical temperature 

 of carbonic acid," new conditions of matter supervene, which cannot 

 be referred either to the liquid or gaseous states, but connect those 

 states by an unbroken continuity with one another. If the temperature 

 or pressure be varied by i or 2 when the carbonic acid is at the 

 critical point, you will have flickering movements of the same kind as 

 those which every one has seen on a fine summer's day, or still 

 better in a telescope, produced by the changing densities of the air, but 

 here those conditions are so intense and extraordinary that you 

 would not be able, I believe, to see one foot before you. You would 

 be in a transparent atmosphere, but at the same time it would be 

 rendered useless for all ordinary purposes by the intensity of these 

 conditions. I may just further mention that the experiments of which 

 you see a representation on the diagram, are experiments which have 

 been carried on with exact measurements to pressures corresponding 

 to the depth of 9000 feet below the level of the sea ; and in the same 



