SCIOPTICON MANUAL.
175
able beat is produced in making the mixture, and for
this reason it should be made beforehand in an earthen¬
ware, not a glass vessel, so as to allow sufficient time for
it to become cool before being added to the granulated
zinc contained in the generator a (Fig. 47).
b
About two pounds of zinc is introduced into the
generator (a glass bottle to hold five gallons or more,
or a vessel made of lead), the top of which, bearing the
tube funnel and bent exit tube, is then replaced, and the
joint being made airtight (in the case of a leaden gene¬
rator by means of a screw, and in the ease of a glass
bottle, by a metallic stopper coated with rubber), the
diluted acid is poured down the long tube funnel b, the
end of which descends far enough into the liquid to
prevent the return of gas in that direction. A brisk
action ensues, the gas effervescing like so much soda-
water. The first portions should, however, be allowed
to escape for some minutes at the outlet of the wash
bottle c, to expel the air. To ascertain when hydrogen
begins to flow, we might apply a light to soap-bubbles