37. I have spoken about touch-paper, but I do not think that I have yet
described its preparation. This is very simple. Obtain some thin blue
paper—not so thin as tissue-paper, but thinner than the ordinary blue
paper used by shopkeepers—brush or sponge this over with a weak solution
of nitre. If this solution is made too strong, the nitre will
recrystallise as the water evaporates in drying, and this should not be
the case.