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.