VI. On the formation of definite figures by the deposition of dust

Abstract

When trying some experiments which had an object other than that described in the following communication, it was noticed that a fine powder when allowed to settle on a slightly warmed plate produced figures which were remarkably clear and definite. So striking and peculiar were these figures, and so simple were the conditions of their formation, that a careful study of them was undertaken. These figures are so clear and sharp that it is easy to obtain exact photographic records of them, an important point, for, at present, it does not seem possible to offer a simple explanation of the complicated relationships which exist between the external conditions and the figures formed. Sensitive as these figures are to outside influences, the forms they assume are very characteristic of different conditions, are perfectly constant, and are easily produced. The general method of obtaining these figures is as follows:—The plate on which the figure is to be deposited is best supported on three pins about 1½ to 2 inches high, and the dust most convenient to use is that made by burning magnesium ribbon. It is kindled and allowed to burn in a receiver. A circular glass dish with straight sides, about 4 inches high and 9 inches in diameter, is a convenient form of vessel to use; and if the vessel be large enough (there should be about 2 inches between the plate and the inside of the receiver); the shape and the material of this dust containing vessel is not of much consequence. After the magnesium has burnt out, this receiver is allowed to stand for a minute or so, and it is then placed over the plate on its stand and allowed to remain there for six to seven minutes. On removing it a clear and definite figure will be found to have formed on the plate. If the plate has been a square one, then a cross consisting of four rays, each starting from a corner and meeting, but not necessarily joining, in the centre, is produced. If the corners be varnished or covered by a small piece of tinfoil (fig. 1) the cross is still formed.