some soap-suds. I have an india-rubber tube connected with the hydrogen
generator, and at the end of the tube is a tobacco-pipe.

[Illustration: Fig. 18.]

I can thus put the pipe into the suds, and blow bubbles by means of the
hydrogen. You observe how the bubbles fall downwards when I blow them with
my warm breath; but notice the difference when I blow them with hydrogen.
[The Lecturer here blew bubbles with hydrogen, which rose to the roof of
the theatre.] It shews you how light this gas must be in order to carry
with it not merely the ordinary soap-bubble, but the larger portion of a
drop hanging to the bottom of it. I can shew its lightness in a better way
than this; larger bubbles than these may be so lifted up; indeed, in
former times balloons used to be filled with this gas. Mr. Anderson will
fasten this tube on to our generator, and we shall have a stream of
hydrogen here with which we can charge this balloon made of collodion. I
need not even be very careful to get all the air out, for I know the power
of this gas to carry it up. [Two collodion balloons were inflated, and
sent up, one being held by a string.] Here is another larger one made of
thin membrane, which we will fill and allow to ascend. You will see they
will all remain floating about until the gas escapes.

What, then, are the comparative weights of these substances? I have a
table here which will shew you the proportion which their weights bear to
each other. I have taken a pint and a cubic foot as the measures, and have
placed opposite to them the respective figures. A pint measure of this
hydrogen weighs three-quarters of our smallest weight (a grain), and a
cubic foot weighs one-twelfth of an ounce; whereas a pint of water weighs
8,750 grains, and a cubic foot of water weighs almost 1,000 ounces. You
see, therefore, what a vast difference there is between the weight of a