_To make a Colour that will appear or disappear by Means of the Air._
Take a smelling-bottle; put in it some alkali volatile, in which you
have dissolved some copper filings: this will produce a blue colour.
Present then the smelling-bottle to one of the company, desiring him to
stop it; and, to their great astonishment, the colour will disappear
as soon as the smelling-bottle is stopped: you will make it easily
re-appear by taking off the cork, which will be not less surprising.