286 



Dr. W. J. Russell. On the Formation of [Jan. 29, 



With regard to the plate on which the figure is deposited, its com- 

 position is not of importance except as a back ground for the dust. 

 X glass plate for many reasons is best, but the figures form with equal 



certainty and sharpness on one of copper, or mercury, or ebonite, or 

 India-rubber, or card-board, &c. In order to heat the plate it may be 

 passed several times over the flame of a lamp, warming it as imiformly 

 as possible, and, if it be a glass plate, until the moisture condensed on 

 the under side has disappeared ; or the plate may be heated by laying- 

 it on a copper plate heated to about 120°C. for 30 minutes, or it 

 may simply be warmed in an air or water bath. The plate is best 

 supported on three pieces of wire about 1 J inches long, and a receiver 

 filled with the dust, inverted over it and allowed to remain there for 

 6 or 7 minutes. 



In order to obtain symmetrical figures the plate on which they 

 are deposited must be perfectly horizontal, and as they are very 

 sensitive to heat, there must be no unequal heating either of the plate 

 or the surrounding atmosphere while the deposition is taking place. 



As long as the plate and the surrounding atmosphere are nearly' of 

 the same temperature only very imperfect figures form, but as the tem- 

 perature rises a more and more nearly perfect figure appears. If the 

 plate be above 17°, indications of pictures are produced when the plate 

 is at a slightly lower temperature than the surrounding atmosphere, but 

 when the difference is 6 D or more, these indications cease altogether. 

 Very good pictures are produced by having the plate at 12° or more 

 degrees above the dust-laden air, and even when the plate is 100° or 

 120° above the air, distinct but thin pictures are produced. The 

 effect of a slight heat below the plate, while the deposit is taking 

 place, is shown to thicken the figure, and distort it in a curious manner 

 and is illustrated by photographs. Also the effect of radiant heat on 

 these figures is shown by the action of a Bunsen burner at distances of 

 12 and 26 inches, and of other sources of heat at considerable 

 distances from the plate. Some singular and complicated effects are 



