An alchemist in his study, discovering phosphorus; a young man and a boy in the background. Mezzotint by W. Pether, 1775, after J. Wright of Derby, 1771.

The procedure is based on a description in an English translation by Andrew Reid of a book by by Pierre Joseph Macquer, Elements of the theory and practice of chymistry, London: Printed for A. Millar, and J. Nourse etc., 1758. It is summarised by Nicolson (op. cit., p. 119) as follows: "The materials from which the phosphorus is prepared [i.e. urine] are heated in a furnace and the phosphorus distilled over into a glass receiver. The end of the retort protrudes from the furnace, and the receiver, known as a balloon, is luted to the retort. The balloon is nearly half filled with water, and as the phosphorus distils over, the upper part is filled with luminous vapour; while from a small hole in the top near the neck of the balloon, the vapour rises in a luminous jet."