DEFINITE FIGURES BY THE DEPOSITION OF DUST. 
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made warmer and warmer, and the surrounding atmosphere kept nearly at the same 
temperature, then the figure gradually alters and becomes more perfect. If the 
difference of temperature between plate and atmosphere be about 5 , there is only a 
small amount of deposit on the central part of the plate, and the four rays are well 
developed. When the difference of temperature is about 12°, then a good clear cross is 
formed, its only imperfection being a slight fuzz in the centre (fig. 8). At a difference 
of temperature of 100° or 120°, the same figure, a cross, is formed, ljut tlie amount of 
dust deposited is less than at lower temperatures. Hence, whether the difference of 
temperature between plates and atmosphere be very considerable or very slight, the 
same effect is produced. A thick piece of glass held in the hand for 30 seconds and 
then placed in the dust atmosphere will have a figure deposited upon it, hut the 
amount of deposit will be small and the figure faint. The figures form best between 
certain limits of temperature, and when there is a marked difference hetveen tlie 
plate and the surrounding atmosphere. They are very sensitive to change of 
temperature ; in fact, to get a perfect cross or other figure, both plate and atmosphere 
must each be uniformly heated. If, in addition to uniformly heating a plate, a 
warmed body he placed below it and kept there during the time that the dust is 
depositing, there is a considerable increase in the amount of deposit and a modification 
of the figure formed; for instance, if a copper cylinder, 12 millims. in diameter and 
14 millims. high, heated to 55°, be placed 30 millims. below the centre of a square 
plate, then the figure shown in fig. 9 is produced. If a piece of glass be only warmed 
by holding it in the hand, and is then placed immediately below the plate, but not 
touching it, a marked and peculiar effect on the cross is produced, as seen in fig. 10. 
If this heating below the plate be increased, either by raising the temperature of the 
small copper cylinder, or by using taller cylinders, so as to bring the source of heat 
nearer to the plate, the amount of deposit is increased, and ultimately tlie figure of 
