4,6 1 
vegetable and animal substances. 
which close the ends of the semi-cylinder and tin-plate prism, 
rise up a few inches to screen the pneumatic apparatus from 
the heat. A third occasional screen of tin-plate is hung on 
at f. All these are furnished with slits for the passage of 
the glass tube. This is made of crown glass, and is generally 
about 9 or 10 inches long, and ^ of internal diameter. 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 at e. By this 
arrangement, should the collar be not absolutely air-tight, 
the pressure of the column of mercury causes the atmo- 
spheric 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 e, is always left above the 
surface of the mercury, the quantity of organic matter em- 
ployed being such as to produce from 6 to 7 cubic inches of 
gaseous product, the volume of the graduated receiver being 
7 cubic inches. 
As the tubes with which I operate have all the same ca- 
pacity, viz. half a cubic inch ; and as the bulk of materials is 
the same in all the experiments, one experiment on the ana- 
lysis 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 tem- 
perature, 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 
* If a be the capacity of the graduated receiver, and b the spare capacity of the 
tubes, then the above volume is b - — • 
d -f- b 
