Continuity of the Gaseous and Liquid States of Matter. 151 



acid ceases to liquefy by pressure he designates the critical point, and 

 he finds it to be 30°- 92 C. Although liquefaction does not occur 

 at temperatures a little above this point, a very great change of 

 density is produced by slight alterations of pressure, and the flick- 

 ering movements also described in 1863 come conspicuously into 

 view. In this communication, the combined effects of heat and 

 pressure upon carbonic acid at temperatures varying from 13° C. to 

 48° C, and at pressures ranging from 4y to 109 atmospheres, are 

 fully examined. 



At 13°* 1 C, and under a pressure, as indicated approximately by 

 the air manometer, of 4889 atmospheres, carbonic acid, now just on 

 the point of liquefying, is reduced to -g-J-.-g- of the volume it occupied 

 under one atmosphere. A slight increase of pressure, amounting to 

 ■£$ of an atmosphere, which has to be applied to condense the first 

 half of the liquid, is shown to arise from the presence of a trace of 

 air (juVd P art ) m the carbonic acid. After liquefaction, the volume 

 of the carbonic acid, already reduced to about ~\^ of its original 

 volume, continues to diminish as the pressure augments, and at a 

 much greater rate than in the case of ordinary liquids. Similar 

 results were obtained at the temperature of 21 c, 5. A third series of 

 experiments was made at 3l°'l, or o, 2 above the critical point. In 

 this case the volume of the carbonic acid diminished steadily with the 

 pressure, till about 74 atmospheres were attained. After this, a rapid 

 but not (as in the case of liquefaction) abrupt fall occurred, and the 

 volume was diminished to one-half by an additional pressure of less 

 than two atmospheres. Under a pressure of 75*4 atmospheres, the 

 carbonic acid was reduced to - 3 -^ T of its original volume under one 

 atmosphere. Beyond this point it yielded very slowly to pressure. 

 During the stage of rapid contraction there was no evidence at any 

 time of liquefaction having occurred, or of two conditions of matter 

 being present in the tube. Two other series of experiments were 

 made, one at 32°'5, the other at 35 0, 5, with the same general re- 

 sults, except that the rapid fall became less marked as the tem- 

 perature was higher. The experiments at 35°* 5 were carried as far 

 as 107 atmospheres, at which pressure the volume of carbonic acid 

 was almost the same as that which it should have occupied if it had 

 been derived directly from liquid carbonic acid, according to the law 

 of the expansion of that body for heat. 



The last series of experiments was made at 48°'l, and extended 

 from 62*6 to 109'4 atmospheres of pressure. The results are very 

 interesting, inasmuch as the rapid fall exhibited at lower tempera- 

 tures has almost, if not altogether, disappeared, and the curve re- 

 presenting the changes of volume approximates closely to that of a 

 gas following the law of Mariotte. The diminution of volume is at 

 the same time much greater than if that law held good. 



The results just described are represented in a graphical form in 

 the figure (p. 152). Equal volumes of air and carbonic acid, mea- 

 sured at 0° C. and 760 millimetres, when compressed at the tempera- 

 tures marked on each curve, undergo the changes of volume indicated 

 by the form of the curve. The figures at the top and bottom indi- 



