246 OXYGEN. 



water is retained in a bird's drinking glass. But when the lead 

 tube is introduced into the gas-holder by this opening, water 

 escapes by it, in proportion as gas is thrown into the cylinder, 

 and rises in bubbles to the top. The progress of filling the 

 gas-holder may be observed by the glass gauge tube g, which is 

 open at both ends, and connected with the top and bottom of 

 the cylinder, so that the water stands at the same height in the 

 tube as in the cylinder. Convenient dimensions for the cylin- 

 der itself are 16 inches in height by 12 in diameter; to fill 

 which a charge of three pounds of manganese may be used. 

 The gauge tube is so apt to be broken, or to occasion leakage 

 at its junctions with the cylinder, when the latter is large, and 

 unwieldy, that it is generally better to forego the advantage 

 it offers, and dispense with this addition to the gas-holder. When 

 applied to a small gas-holder, the ends of the tube are conveniently 

 adapted to the openings of the cylinder, by means of perforated 

 corks, which are afterwards covered with melted sealing-wax. 



After the cylinder is filled, the lower opening by which the 

 gas was admitted is closed by a good cork, or by a brass cap 

 made to screw over it. The superior cylinder is an open water 

 trough, connected with the inferior cylinder by two tubes pro- 

 vided with stopcocks, m and n, one of which m is continued to 

 the bottom of that vessel, and conveys water from the su- 

 perior cylinder, while the other tube n, terminates at the top 

 of the inferior cylinder, and affords a passage by which the gas 

 can escape from it, when water is allowed to descend by the other 

 FTG. 31. tube. The tube and perforation of the stopcock of m 

 should be considerably wider than n. A jar a is filled 

 with gas by inverting it full of water in the superior 

 cylinder, over the opening of n, as exhibited in the 

 figure, and allowing the gas to ascend from the infe- 

 rior cylinder. Gas may likewise be obtained by the 

 stopcock / (Figure 30) water being allowed to enter 

 by m at the same time. 



Oxygen may likewise be disengaged from oxide of manga- 

 nese in a flask or retort, by means of sulphuric acid diluted 

 with an equal bulk of water, but this is not a process to be 

 recommended. When only a small quantity of oxygen is re- 

 quired, it is better to have recourse to chlorate of potash, 

 which has also the advantage of giving a perfectly pure 

 gas. 



