Experiment 22.--Pass a d.t. from a H generator to the top of a
receiver or t.t. (Fig. 13). The escaping H being so much lighter
than air will force the latter down. To obtain the gas unmixed
with air, the d.t. should tightly fit a cardboard placed under
the mouth of the receiver. When filled, the receiver can be
removed, inverted as usual, and the gas tested. In this and other
experiments for generating H, a thistle-tube, the end of which
dips under the liquid, can be used for pouring in acid, as in
Figure 13.

35. Philosopher's Lamp and Musical Flame.

Experiment 23.--Fit to a cork a piece of glass tubing 10 or 15
cm. long, having the outer end drawn out to a point with a small
opening, and insert it in the H generator. Before igniting the
gas at the end of the tube take the, precaution to collect a t.t.
of it by upward displacement, and bring this in contact with a
flame. If a sharp explosion ensues, air is not wholly expelled
from the generator, and it would be dangerous to light the gas.
When no sound, or very little, follows, light the escaping gas.
The generation of H must not be too rapid, neither should the
t.t. be held under the face, as the cork is liable to be forced
out by the pressure of H. A safety-tube, similar to the thistle-
tube above, will prevent this. This apparatus is called the
"philosopher's lamp." Thrust the flame into a long glass tube 1-
1/2 to 3 cm. in diameter, as shown in Figure 14, and listen for a
musical note.

36. Product of Burning H in Air.