DEFINITE FIGURES BY THE DEPOSITION OF DUST. 
187 
or fumes of ammonium chloride. In fact, the necessary condition is tliat the 
dust be very fine, then always the same figure is formed. Tlje pr(.)duct iormed 
by burning magnesium is, however, the best form of dust to use. It is easily 
obtained, and has a silvery whiteness in appearance, which gives distinctness to the 
figures. With regard to the plate on which the figure is to form, its composition, 
like that of the dust, is of no importance; the shape determines the figure, not its 
constitution. Glass, for several reasons, is the best material for the plate, but copper, 
zinc, silver, antimony or other metal may be used, or ebonite, celluloid, black india- 
rubber, cardboard, &c., in fact, the receiving surface is not necessarily a solid 
substance; mercury in a square vessel will have deposited on it a figure similar to 
that on a piece of glass of the same size and shape, and, still more, the surface of the 
glass plate may be coated with oil, gum, copal-varnish, &c., and the cross will form 
as if they were not present. Obviously, with regard to the visibility of the figures 
formed, the nature of the plate is of considerable importance ; on some substances the 
figures are more easily seen than on others. In the following experiments glass 
plates have been used, except when mention is made to the contrary. 
Passing from the materials used to the active agent in producing the figures, 
namely heat, it should be stated that there are many different ways of applying it, 
and different results are produced. The simplest way is to pass the plate two or three 
times over the flame of a small Bunsen or spirit lamp. If it be a glass plate, a good 
indication of sufiicient heating is when the condensed moisture disappears it is of 
little importance whether the heated side or the other one is uppermost then the 
plate is enveloped in the dust atmosphere by placing the receiver, filled with dust, 
over it, and leaving it there for the six or seven minutes. To obtain a figure in its 
simplest form and as dense and clear as possible, it is necessary that the plate lie 
equally warmed all over; a convenient way of doing this is to lay the plate on one or 
copper, heated to about 20° C., for about half a minute, or an ordinary air or watei' 
liath will answer the same purpose. As long as the j^late and the surrounding dust 
atmosphere have approximately the same temperature, the deposit formed is nearly 
uniform; there is only a slight appearance of any figure, but as soon as any rise ot 
temperature occurs, then a figure begins to appear. At first the indications are very 
slight, and occur only round the edge of the plate; but as the temperature is raised, 
the figure spreads over the whole of it. A figure may also be developed by having 
the plate at a lower temperature than that of the surrounding atmosphere, provided 
that the plate is not below 17° C., but the figures produced in this way are sliglit and 
imperfect and disappear altogether when the plate is 6° below th-"'t of the atmosphere. 
In order to determine roughly the temperatures of the plate and its surrounding 
atmosphere, a receiver, of the same shape and size as the glass one, was made of 
asbestos cloth and covered with cardboard ; in the top of it a hole was made, and a 
delicate thermometer introduced. A few of the results obtained will show the 
nature of the alterations produced by differences of temperature lietween plate and 
2 B 2 
