1869.] 



Gaseous and Liquid States of Matter. 



43 



marcation" between the liquid and gas became fainter, lost its curvature, 

 and at last disappeared, the tube being then filled with a fluid which, from 

 its optical and other properties, appeared to be perfectly homogeneous. 

 The present paper contains the results of an investigation of this subject, 

 which has occupied the author for several years. The temperature at 

 which carbonic 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 flickering 

 movements, also described in 1863, come conspicuously into view. In 

 this communication, the combined effects of heat and pressure upon car- 

 bonic acid at temperatures varying from 13° C. to 48° C, and at pres- 

 sures ranging from 4a to 109 atmospheres, are fully examined. 



Atl3°*l C.j and under a pressure, as indicated approximately by the 

 air manometer, of 48 89 atmospheres, carbonic acid, now just on the point 

 of liquefying, is reduced to y-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 ( T1 j\ ri j part) 

 in the carbonic acid. After liquefaction, the volume of the carbonic acid 

 already reduced to about of its original volume, continues to dimi- 

 nish 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°*5. A third series of experiments was made at 3l°*l, or 0 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 car- 

 bonic acid was reduced to .. | 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°*5, with the same general results, except that the rapid fall became 

 less marked as the temperature was higher. The experiments at 3o°-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°*1, and extended from 

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

 inasmuch as the rapid fall exhibited at lower temperatures has almost, if 

 not altogether, disappeared, and the curve representing the changes of 

 volume approximates closely to that of a gas following the law of Mariotte. 



