The very beautiful experiment of Mr Sang, communicated to the Society on the 7th April, 1873, suggested to me, as soon as I heard him read his description of it, an explanation which was confirmed by a subsequent examination of his specimens. Some remarks made to me by members of the Council of the Society, three days afterwards, led me to write, and deposit (under seal, as Mr Sang had announced that he was still prosecuting his inquiry) with the Secretary the following hastily written document, which has been since that time in his possession, and is now printed verbatim.