Continuity of the Gaseous and Liquid States of Mattel' . 153 



versally regarded as the gaseous to what is, in like manner, univer- 

 sally regarded as the liquid state. As a direct result of his experi- 

 ments, he concludes that the gaseous and liquid states are 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 conti- 

 nuity. From carbonic acid as a perfect gas, to carbonic acid as a 

 perfect liquid, the transition 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 a 

 state of instability, and suddenly passes, without change of pressure 

 or temperature, but with the evolution of heat, to the condition 

 which, by the continuous process, can only be reached by a long and 

 circuitous route. 



The author discusses the question as to what is the condition or 

 state of carbonic acid when it passes, at temperatures above 31°, from 

 the ordinary gaseous state down to the volume of the liquid with- 

 out giving any evidence during the process of the occurrence of lique- 

 faction, and arrives at the conclusion that the answer to this ques- 

 tion is to be found in the intimate relations which subsist between 

 the gaseous and liquid states of matter. In the abrupt change which 

 occurs when the gases are compressed to a certain volume at tempe- 

 ratures below the critical point, molecular forces are brought into 

 play which produce a sudden change of volume ; and during this 

 process it is easy to distinguish, by optical characters, the carbonic 

 acid which has collapsed from that which has not changed its vo- 

 lume. But when the same change is effected by the continuous pro- 

 cess, the carbonic acid passes through conditions which lie between 

 the ordinary gaseous and ordinary liquid states, and which we have 

 no valid grounds for referring to the one state rather than to the 

 other. 



Nitrous oxide, hydrochloric acid, ammonia, sulphuric ether, sul- 

 phuret of carbon, all exhibited critical points when exposed under 

 pressure to the required temperatures. 



The author proposes for the present arbitrary distinction between 

 vapours and gases, to confine the term vapour to gaseous bodies at 

 temperatures below their critical points, and which therefore can be 

 liquefied by pressure, so that gas and liquid may exist in the same 

 vessel in presence of one another. 



The possible continuity of the liquid and solid states is referred 

 to as a problem of far greater difficulty than that which forms the 

 subject of this communication, and as one which cannot be resolved 

 without careful investigation. 



