NATURAL PHILOSOPHY. 131 



abrupt alteration of volume or sudden evolution of heat, it ends 

 with a liquid. The closest observation fails to distinguish any- 

 where indications of a change of condition in the carbonic acid, 

 or evidence at any part 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 ebulli- 

 tion on the removal of the pressure. For convenience, this pro- 

 cess 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 70 atmospheres 

 when the carbonic acid had cooled to 31. 



"We are now prepared for the consideration of the following 

 important question : A\ r hatis 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 point 

 of the process of liquefaction having occurred ? Does it continue 

 in the gaseous state, or does it liquefy, or do we have 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 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 and nitrogen. On the other hand, 

 when the experiment is made with carbonic acid at temperatures a 

 little above 31 C., the great fall that occurs at one period of the pro- 

 cess 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 dimi- 

 nution 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 al- 

 ready described, is to be found in the close and intimate 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 car- 

 bonic 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 



