Caution.
—Do not bring a flame anywhere near the apparatus and as a further precaution wrap a thick towel around the hydrogen flask.
The bubbles that are blown rise in the air because both the hydrogen and the oxygen are lighter than the air.
The Uncanny Wheel.
—The Effect.—A pitcher is shown full of emptiness and then a cardboard wheel, 4 inches in diameter, with buckets, or cones 1 inch high and ³⁄₄ inch across glued to the rim and which is mounted on a wire so that it can be revolved, is passed for examination.
Placing the wheel on the table you hold the empty pitcher above it and pour out nothing on it when the wheel will turn round just as though you were pouring water on it. It is indeed uncanny. The idea is shown at [A in Fig. 121].
The Cause.
—But it is all canny enough when you know how it is done. While the pitcher is apparently empty you have, forsooth, previously filled it with a gas called carbon dioxide. This gas is 1¹⁄₂ times as heavy as air.
The cardboard wheel does not move in the air because the latter pushes on all parts of it equally. When, however, you pour the carbon dioxide gas on it from the pitcher, since it (the gas) is heavier than the air it fills the little buckets and makes them heavier just as surely as if you poured water on them; and hence the wheel revolves.
