It could not be a lunar arch, the moon being then many degrees below the horizon, and the arch in a place, where it could not be affected by the moon’s rays. The consciousness of my inability to give a proper account of such an uncommon appearance could not deter me from the attempt. I think I have said all that is necessary on this subject; yet am ready to answer any question for the farther illustrating of it. I am, Reverend Sir, Your most humble Servant, Geo. Edwards. College of Physicians, London, June 6th, 1757. XXXVI. _The Effects of the_ Opuntia, _or Prickly Pear, and of the_ Indigo _Plant, in colouring the Juices of living Animals. Communicated by_ H. Baker, _F.R.S._ [Read June 23, 1757.] June 23d, 1757. MR. Baker received a letter yesterday from Dr. Alexander Garden, of Charles Town in South Carolina, part of which he hopes he shall be excused for laying before the Royal Society. The Doctor writes thus:----“As you desired, I tried the effects of the prickly pear in colouring the urine. A few days after your letter, I went down to one of the islands, and gathered some of the fruit, and gave four of the pears to a child of three years of age, and six pears to one of five. The next morning I examined the urine of both, and it appeared of a very lively red colour, as if tent-wine had been mixed with clear water. The urine of the eldest was deeper coloured, and of a darker look: the youngest (who always naturally made clear urine) was of a more lively and beautiful red. Next day I gave six pears to a Negroe wench, who gave suck, and strictly forbad her suckling her child for six or eight hours; and then taking some of her milk in a tea-cup, and setting it by for some hours, the cream had a reddish lustre, tho’ it was very faint. I was led to this last experiment by an observation, which I made on the milk of cows, who had fed in an indigo-field; the indigo had not only tinged their urine blue, but the cream of the milk was of a most beautiful blue colour, and had a radiated appearance from the centre (Is it not hence probable, that the dye is the oily part of the plant?). The milk underneath was clear and white as usual.” Dr. Garden wrote, a year ago, that the prickly pear grows in great abundance about Carolina; and also that the cochineal insects are found upon it; but hitherto no attempts have been made to cure them as the Spaniards do. In hope, that some rich dye may be produced from the plant itself, Mr. Baker proposed some experiments to Dr. Garden, which he intends to prosecute this summer. XXXVII. _Account of an extraordinary Shower of black Dust, that fell in the Island of_ Zetland _20th_ October 1755[194]. _In a Letter from Sir_ Andrew Mitchell, _of_ Westshore, _Bart. to_ John Pringle, _M.D. F.R.S._ [Read June 23, 1757.]