1869.] 



Gaseous and Liquid States of Matter. 



45 



sures in atmospheres ; the volumes of the gas and air are measured up- 

 wards from the dotted horizontal line. 



The author has exposed carbonic acid, without making precise measure- 

 ments, to higher pressures than any of those mentioned, and has made it 

 pass, without breach of continuity, from what is universally regarded as 

 the gaseous to what is, in like manner, universally regarded as the liquid 

 state. As a direct result of his experiments, 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 grada- 

 tions 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 may be accomplished by a con- 

 tinuous process, and the gas and liquid are only distant stages of a long 

 series of continuous physical changes. Under certain conditions of tem- 

 perature and pressure, carbonic acid finds itself, it is true, in a state of 

 instability, and suddenly passes, without change of pressure or tempera- 

 ture, but with the evolution of heat, to the condition which, by the con- 

 tinuous 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 ordi- 

 nary gaseous state down to the volume of the liquid, without giving any 

 evidence during the process of the occurrence of liquefaction, and arrives 

 at the conclusion that the answer to this question 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 temperatures 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 

 volume. But when the same change is effected by the continuous process, 

 the carbonic acid passes through conditions which lie between the ordi- 

 nary 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, sulphuret 

 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 pres- 

 sure, 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 in- 

 vestigation. 



