350 Liquefaction and Solidification of Carbonic Acid. 



at J ; so that when the receiver is being separated from the gen- 

 erator, the contents of both may be retained. The stop-cocks in 

 common use are inadequate to resist the pressure, and therefore a 

 screw stop-cock is indispensable. It is made to close a small 

 aperture by means of a conical point, and having a double cone, 

 it closes an outlet also when the cock is completely open, so as 

 to prevent the escape of gas by the sides of the screw. 



The receiver, F, is of the capacity of about a pint. The pipe, 

 G, G, turned at a right angle at G, descends so as almost to touch 

 the bottom of the cavity in F. The stop-cock H, G, is similar 

 to I and J. L is a glass tube connected at each end to a socket 

 of brass, which communicates with the interior of F. It is the 

 gauge for observing the level of the liquid in F. 



The gauge for measuring the pressure is peculiar. Into a 

 wrought iron box, S, are inserted, by screws, two sockets, T and 

 U. The former descends almost to the bottom of the box, which 

 is nearly filled with mercury. Through the axis of the screw, 

 X, a small tube passes into the cavity of S, and is continued to 

 the top of it, so as to rise above the mercury. Two strong ba- 

 rometer tubes, R and M, are cemented* into U and W, and her- 

 metically sealed at the upper ends. These tubes are carefully 

 graduated. In one of them, U, a short cylinder of mercury is 

 made to stand at Y at the commencement of the experiment. 

 The other, socket and all, is full of air, as no mercury is intro- 

 duced into it. A very fine screw at W, enables the operator to 

 regulate the quantity of air in T. 



The tin cup, O, used to collect the solid acid, is covered by a 

 lid, Z, perforated by a pipe, P, whose top is full of small holes. 

 The handle Q., is hollow, so as to fit the end of the pipe of the 

 receiver at G. To secure the ,hand of the operator from the cold 

 produced by the experiment, the handle is carefully wrapped up 

 in some kind of cloth. 



The apparatus is prepared for use by removing the screw E, 

 and placing If lbs. of bicarbonate of soda in the generator. A, 



* The cement used was made of shell lac 3 or 4 parts, white or crude turpen- 

 tine 1 part, melted at as low a temperature as possible, so as not to make bubbles 

 in the mixture. This cement is very strong, but liable, without great care in the 

 regulation of the heat, to have capillary tubes in it from the vaporization of the 

 turpentine. This defect may be completely corrected by cutting away, when 

 cold, the external mass of cement, and putting on a little common cap cement, 

 which melts at a much lower temperature and closes the tubes. 



