S552 Mr Kerr on a nexv Form of Bent Tubes y 
part of the second branch. As the gas accumulates in this 
branch, it displaces the liquid, which ascends in the other. The 
descending part of the second branch, however, will still remain 
filled with liquid, which is now to be 'run back into ijie- first 
branch, by elevating the shut end of the tube somewhat higher 
than the second angle. In this manipulation, the spilling of li- 
quid from the open mouth of the first branch, or the introduc- 
tion of atmospheric air into the gas, through the same branch, 
may be avoided, because the second is greater than the first angle 
of the tube *. 
When the operator wishes to collect any of the gas in a sepa- 
rate vessel, he dips the inferior end of the descending part of 
the second branch beneath the surface of water, or mercury, ac- 
cording to the nature of the gas evolved. The stopper or cork 
D is then to be taken out, when the gas will issue from the 
mouth of the tube. When a sufficient quantity of gas has been 
collected, the stopper or cork is replaced. 
Several tubes, such as have been just described, may be placed 
in longitudinal slits made in a wooden board, which, for the sake 
of convenience, should be attached in a horizontal position to the 
upper edge of one of the sides of an oblong pneumatic trough. 
A trough and shelf of a convenient size, together with one of 
the tubes in its place, are represented in Fig. 8. The internal 
measurement of this trough is 7 inches in depth, the same in 
width, and 10 in length. E is a horizontal shelf, with 4 slits, 
for holding the same number of tubes. The extremities of this 
shelf rest on the end-boards of the pneumatic trough, prolonged 
beyond the trough itself. Within the trough, and on the side 
next the shelf for holding the tubes, another shelf F is placed, 
like the common shelf in a pneumatic trough, at the depth of 
an inch or two. This shelf runs the whole length of the trough, 
viz. 10 inches, is inches broad, and has slits cut in it corre- 
sponding to those in the other shelf. When any of the gas is to 
be transmitted from one of these tubes into a receiver, this is 
* That is, the recurved part C forms a greater angle with the ascending part 
B of the same branch, than the two branches do with each other, though the last 
mentioned angle may be taken for the greatest, because it is more rounded,- — or 
rather, the vertex of this angle is not formed, the two branches being united a little 
higher by a small portion of tube, in a position nearly horizontal. 
