498 



SEC. 13. CHEMISTRY. 



than sufficient to fill the cylinder a. In the iron tube connecting the cylinder 

 and the reservoir is intercalated a stopcock c, by which the connexion 

 between the cylinder and reservoir may at pleasure be made or cut off. In 

 the reservoir b a small iron tube d is inserted, connected by a tube of 

 caoutchouc with a forcing pump firmly fixed to the table on which the 

 apparatus is placed. By means of this forcing pump the air contained in the 

 upper part of the reservoir may be compressed, and any required pressure put 

 upon the mercury contained in it. The cylinder a is cemented by means of 

 a resinous cement into two steel caps, e and/; the lower cap f is screwed 

 firmly upon the support of the apparatus, which is made of iron. The 

 cylinder is connected by means of a channel cut in the lower part of the steel 

 cap f and continued through the iron frame, with a glass tube g, which is 

 about half an inch in diameter and graduated in millimetres. This tube I 

 shall speak of as the pressure tube. The cylinder a and the pressure tube g 

 are thus in permanent connexion, and constitute one vessel, which is broken 

 into parts solely for facility of construction. This apparatus is supported 

 upon three screws, as shown in the figure, by the adjustment of which the 

 pressure tube g is placed in a perpendicular position before the commence- 

 ment of the experiment. The steel stopcock k is a three-way stopcock, in 

 which the channels are so cut that a communication may be made between 

 the tube / and the cylinder a, or between the tube m and the cylinder a, or 

 between the tubes I and m (all other communications being shut off) at 

 pleasure ; or the communications may be entirely closed. 



In the cylinder a is placed a thin piece of glass rod, to which seven points 

 are attached, also of glass, as shown in the figure. The capacity of the 

 cylinder a between each point is ascertained by calibration with mercury. 



The cylinder and pressure tube are enclosed in a second glass cylinder n 

 filled with water, in which a thermometer is placed. 



Eor further detail as to the construction, calibration, and mode of operating 

 with the apparatus, see Phil. Trans., 1872, p. 3. 



In Fig. 5 a drawing is given of the whole apparatus as arranged for 

 experiment. 



Fie .5. 



