130 ANNUAL OF SCIENTIFIC DISCOVEKY. 



until a pressure of 60 atmospheres is reached. A slight deviation 

 from perfect abruptness in the fall is shown by Dr. A., to result 

 from the presence of a small quantity of air in the carbonic acid 

 tube. The curve for 31.1 C. shows no such abrupt fall, bub a rapid 

 descent between the pressures of 73 and 75 atmospheres. This 

 descent becomes gradually less marked as the temperature rises, 

 and when a temperature of 48.1 C. has been obtained, it has 

 almost, if not altogether, disappeared. 



At any temperature between 57 C., and 30.92 C., carbonic 

 acid, under the ordinary atmospheric pressure, is unquestionably 

 in the state of a gas or vapor. On augmenting the pressure, 

 the volume diminishes at a more rapid rate than that of a perfect 

 gas, till liquefaction begins, a sudden diminution of volume then 

 taking place. With some care it is possible to arrange the ex- 

 periment so that part of the carbonic acid shall be in the liquid, 

 and part in the gaseous state ; the carbonic acid thus coexisting 

 in two distinct physical conditions in the same tube and under the 

 same external pressure. The result is, however, far different if 

 the experiment be made at 30.92 C. or at any higher tempera- 

 ture. At this temperature, and under a pressure of about 74 at- 

 mospheres, the density of liquid and gaseous carbonic acid, as 

 well as all their other physical peculiarities, are absolutely identi- 

 cal, and the most careful observation fails to discover any hetero- 

 geneity at this or higher temperatures in carbonic acid, when its 

 volume is so reduced as to occupy a space in which at lower tem- 

 peratures a mixture of gas and liquid would have been formed. 

 The carbonic acid has, in fact, become one homogeneous fluid, 

 which cannot by change of pressure be separated into two dis- 

 tinct conditions. This temperature of 30.92 C., Dr. Andrews 

 calls the critical point of carbonic acid. Other gases show the 

 same phenomena, but have different critical points. The rapid 

 changes in density, which slight changes of temperature or pres- 

 sure produce, when the gas is reduced at temperatures a little 

 above the critical point, to the volume at which it might be ex- 

 pected to liquefy, account for the flickering movements referred 

 to in the beginning of this article. 



" I have frequently exposed carbonic acid," says Dr. Andrews, 

 "without making precise measurements to much higher pres- 

 sures than any marked in the tables, and have made it pass, 

 without break or interruption, from what is regarded by every one 

 as the gaseous state, to what is in like manner universally re- 

 garded as the liquid state. Take, for example, a given volume 

 of carbonic acid gas at 50 C., or at a higher temperature, and ex- 

 pose it to increasing pressure till. 150 atmospheres have been 

 reached. In this process the volume will steadily diminish as the 

 pressure augments, and no sudden diminution of volume without 

 the application of external pressure will occur at any stage of it. 

 When the full pressure has been applied, let the temperature be 

 allowed to fall, till the carbonic acid has reached the ordinary 

 temperature of the atmosphere. During the whole of this opera- 

 tion, no breach of continuity has occurred. It begins with a gas, 

 and by a series of gradual changes, presenting nowhere any 



