1903.] 



Definite Figures by the Deposition of Dust. 



287 



produced by placing a source of heat above the plate instead of below 

 it. A large number of experiments are also recorded and illustrated 

 showing the effect which different bodies in the immediate neighbour- 

 hood of the plate have on the figures which are formed. Taking only 

 one case, that of a pin. When it is placed in contact and at right 

 angles to the plate a definite deposit is produced, and this varies as 

 the pin is moved further and further away, and as it is placed either 

 on a level with plate, or above or below it. Even when the pin is 

 6 mm. below the level of the plate, and 2 mm. away from it, a distinct 

 effect is produced. Again, these dust currents may be influenced in 

 a remarkable way by suspending glasses of different sizes, and at 

 different heights above the plate on which the figures are depositing, 

 and photographs of the figures produced are given. The effects pro- 

 duced by obstructions of different sizes laid on different parts of the 

 plate are also shown. 



It was also found that a current of dust drawn through a tube will 

 form a characteristic figure on a plate, which need not be warmed, as 

 it passes over it. 



If the magnesia dust be allowed to settle on a surface of water y 

 about the temperature of 17° C. or, on water containing a very small 

 amount of alcohol or glycerine, the deposit which forms on the 

 surface breaks up, by the powder sinking, into a figure of cellular 

 form. 



Magnesia dust, which was generally used, undergoes some strange 

 changes. When first deposited it is removed by the slightest touch, 

 but if the plate be kept for a week or fortnight it may then be softly 

 brushed over without damage to the figure. Another change which this 

 powder undergoes is shown by collecting it immediately it forms, and 

 examining it under a microscope, when it will be found to consist of 

 irregular shaped and separate particles, but if the collection of the 

 dust be made a few minutes after its formation, it is then seen that 

 the particles are strung together, forming small and irregular fibres. 

 In the various figures that have been produced the magnesia seems to 

 have assumed this form. 



