I have in my hand a glass cup with a cover of the same material, as
transparent as possible.
I put on the lid, and let this volunteer hold it at a distance from me,
whilst I puff towards it the smoke of this perfumed cigarette.
Keep your eyes on the covered cup, for you will see that the smoke
enters it, though hermetically sealed. To convince you that there is no
ocular illusion, let my obliging Ganymede lift off the cover.
There, away flies the smoke caught in it.
Again close it, while I again despatch more smoke to it. Shall I repeat
the experiment, for I warn you I am prepared to continue till morning.
Three or four times will suffice, eh? So much the better for your
patience.
_Explanation._—Into the cup put a few drops of alkali, and move the
vessel about so that the inside is coated with the liquid; treat the
cover with chloridric acid, in the same way. When these two are brought
into contact by the junction of the cover and vase, a thick vapour is
produced, which resembles tobacco smoke. Take care not to cover the cup
until just when you wish the vapour to appear, as its formation is
instantaneous.
[Illustration: Fig. 44.]