Here is what in former times was called the
"philosopher's candle." It is a little phial with a cork, and a tube or
pipe passing through it.
[Illustration: Fig. 17.]
And I am now putting a few little pieces of zinc into it. This little
instrument I am going to apply to a useful purpose in our
demonstrations--for I want to shew you that you can prepare hydrogen, and
make some experiments with it as you please at your own homes. Let me here
tell you why I am so careful to fill this phial nearly, and yet not quite
full. I do it because the evolved gas, which, as you have seen, is very
combustible, is explosive to a considerable extent when mixed with air,
and might lead to harm, if you were to apply a light to the end of that
pipe before all the air had been swept out of the space above the water. I
am now about to pour in the sulphuric acid. I have used very little zinc,
and more sulphuric acid and water, because I want to keep it at work for
some time. I therefore take care in this way to modify the proportions of
the ingredients, so that I may have a regular supply--not too quick, and
not too slow. Supposing I now take a glass and put it upside-down over the
end of the tube, because the hydrogen is light I expect that it will
remain in that vessel a little while.
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