NATURAL PHILOSOPHY. 129 



tially liquefying carbonic acid by pressure alone, and gradually 

 raising at the same time the temperature to 88 Fahrenheit, the 

 surface of demarcation between the liquid and the gas became 

 fainter, lost its curvature, and at last disappeared. The space 

 was then occupied by a homogeneous fluid, which exhibited, when 

 the pressure was suddenly diminished or the temperature slightly 

 lowered, a peculiar appearance of moving or flickering strife 

 throughout its entire mass. At temperatures above 88, no ap- 

 parent liquefaction of carbonic acid or separation into two distinct 

 forms of matter could be effected, even when a pressure of 300 or 

 400 atmospheres was employed. Nitrous oxide gave analogous 

 results. 1 ' 



In the later series, Dr. A. still made use of carbonic acid. In 

 his apparatus the carbonic acid was contained in a glass tube, 

 capillary in the upper and larger part of its length, and for th<; 

 remainder of the widest bore in which the column of mercury 

 would remain without displacement when the tube was placed iii 

 a vertical position. A movable column or bar of mercury con- 

 fined the gas to be operated on. This glass tube was secured by 

 careful packing in a massive end-piece of brass, which carried a 

 flange by means of which a water-tight junction could be made 

 with a corresponding flange attached to a cold-drawn copper tube 

 of great strength. To the other end of the copper tube a similar 

 end-piece was firmly bolted. The latter carried a fine steel screw, 

 7 inches long, which was packed with such care that the packing 

 was capable of resisting a pressure of 400 atmospheres or more. 

 Before commencing the experiment, the body of the apparatus 

 was filled with water; the upper end-piece, carrying the glass 

 tube in which was the gas to be operated upon, was firmly secured 

 in its place, and the pressure was obtained by screwing the steel 

 screw into the water-chamber. When the gas or liquid was ex- 

 posed to very low temperatures, the end of the capillary-tube was 

 made to dip into a bath of ether and solid carbonic acid under a 

 bell-jar, from which the air could be exhausted. 



In order to estimate the pressure in these experiments, a com- 

 pound form of the apparatus was used, a second tube, containing 

 air, being put in communication with the first, and used as a 

 manometer. In order to keep the tubes at fixed temperatures, 

 each was surrounded with a case, through which a current of 

 water passed, by which the temperature could be regulated. 



The temperature of the water surrounding the air-tube was 

 kept stationary, coinciding with that of the apartment ; that sur- 

 rounding the carbonic acid tube was made to vary from 13.1 Cen- 

 tigrade to 48 Centigrade. .The volumes of both air and gas 

 were carefully read by a cathetometer. 



The observations were recorded by the graphical method, being 

 embodied in a scries of curves. The air-curves are drawn for 

 temperatures of 13.1 C., 31.1 C., and 48.1 C., the ordinates rep- 

 resenting the volumes, and the abscissas the pressures. 



In the carbonic acid curves for 13.1 C. there occurs an abrupt 

 fall (diminution of volume) at a pressure of 49 atmospheres. 

 The curve for 21.5 C. exhibits a corresponding fall, but not 



