D 



346 Dr. Ure on the Ultimate Analysis of [May, 



and a half wide, perforated with holes, and resting on the edge 



of a hollow prism of tin-plate, represented more distinctly in 



fig. 2, where n shows a slit, through 



which the sealed end of the glass tube 



may be made to project, on occasion. . Fig.l 



Fig. 1, i is a handle attached to the se- | n 



micylinder, by which it may be slid back- 

 wards or forwards, and removed at the 

 end of the process, d is a sheath of 

 platinum foil, which serves, by aid of a 

 wire laid across, to support the middle 

 of the tube, when it is softened by 

 ignition. At g the plates which close 

 the ends of the semi-cylinder and tin- 

 plate prism, rise up a few inches to screen the pneumatic appa- 

 ratus from the heat." 



A third occasional screen of tin-plate is hung on for the same 

 purpose. " All these are furnished with slits for the passage of 

 the glass tube. This is made of crown glass, and is generally 

 about nine or ten inches long, and three-tenths of internal dia- 

 meter. It is connected with the mercurial cistern by a narrow 

 tube and caoutchouc collar. This tube has a syphon form, and 

 rises about an inch within the graduated receiver. By this 

 arrangement, should the collar be not absolutely air-tight, the 

 pressure of the column of mercury causes the atmospheric air to 

 enter at the crevice, and bubbles of it will be seen rising up 

 without the application of heat. At the end of the operation, 

 the point of the tube is always left above the surface of the mer- 

 cury, the quantity of organic matter employed being such as to 

 produce from six to seven cubic inches of gaseous product, the 

 volume of the graduated receiver being seven cubic inches." 



" As the tubes with which I operate have all the same capa- 

 city, viz. half a cubic inch ; and as the bulk of materials is the 

 same in all the experiments, one experiment on the analysis of 

 sugar or resin, gives the volume of atmospheric air due to the 

 apparatus, which volume is a constant quantity in the same 

 circumstances of ignition. And since the whole apparatus is 

 always allowed to cool to the atmospheric temperature, the 

 volume of residual gas in the tubes comes to be exactly known, 

 being equal, very nearly, to the primitive volume of atmospheric 

 air left after the absorption of the carbonic acid in the sugar or 

 resin experiment* Thus this quantity, hitherto ill appreciated 

 or neglected in many experiments, though it is of very great 

 consequence, may be accurately found. At k, fig. 1, a little tin- 

 plate screen is shown. It is perforated for the passage of the 



" If a be the capacity of the graduated receiver, and ft the spare capacity of the tubes, 



ft 



then the above volume is ft -. 



a + b 



