xiv 



sure and volume of carbonic acid, communicated to this Society in 

 the Bakerian Lectures of 1869 and 1876. 



He showed that for temperatures below about 30*9° C. as the 

 pressure is increased we come to a point where condensation occurs, 

 where the gas is converted into a liquid. At this pressure there are 

 two limiting values of the volume, one when all the substance is 

 vapour, and one when it is all liquid. Between these the substance 

 is partly vapour and partly liquid, the distinction between the two 

 being visible, as the liquid and the vapour refract light differently, 

 and are separated from one another by a distinct meniscus. As the 

 temperature approaches 30'9° the abrupt change of volume on con- 

 densation becomes less and less in amount, above that temperature 

 (the Critical Temperature of carbonic acid) there is no abrupt change 

 of volume, and no visible condensation. Near 309°, but above it 

 there is a rapid, but not abrupt, change of volume ; as the temperature 

 rises this rapid change of volume becomes less and less marked, and 

 the isothermal approximates more and more to the hyperbolic form. 

 This extraordinary character of the isothermals — discontinuous below 

 a particular temperature, continuous above it — led Andrews to the 

 remarkable discovery which gives the title to his Lectures, " The 

 Continuity of the Gaseous and Liquid States of Matter." 



If we represent the relation of temperature, pressure and volume 

 by means of a surface, where the rectangular co-ordinates x, y, and z 

 correspond to p, t, and v (as was done by Professor James Thomson to 

 illustrate Andrews' work), we see that there is what may be called a 

 cliff, points at the top of which correspond to the substanee at the 

 condensing point, but all in the state of vapour, while points imme- 

 diately below them, at the base of the cliff, correspond to the substance 

 just condensed and all in the state of liquid. The cliff becomes less 

 and less high as x and y increase, and vanishes at the place correspond- 

 ing to the critical point. From a point on the slope above the cliff 

 we can pass to a point on the slope below, either by dropping down 

 the vertical face or by going round the end of the cliff. To quote 

 Andrews' words " . . . . the author has made carbonic acid 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 gradations 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 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 



