DEFINITE FIGUKES BY THE DEPOSITION OF DUST. 
LII3 
altered to a remarkable extent, and which shonld throw light on the mode of their 
formation. It is hy the proximity to the plate of other bodies. For instance, if a 
piece of glass or metal as long as the plate and 10 millims. wide be fixed against it 
so as to project aliove it, then an even deposit forms under its shadow. If lioles are 
cut in this screen, no deposit takes place on tlie glass in front of the holes. Fig. 22 
shows what happened when a scpiare glass had a piece of metal with holes cut in it 
pressed against it. In Iroiit of each opening in the screen there is a cleai space on 
the plate. Another curious, but very complicated efiect is produced hy cutting a 
re-entering angle out of a square of copper. It is difficult to follow how tlie deposit 
can form in the way shown in fig. 22a. 
Bearing on this same point is the fact that If a w^armed plate be placed on the 
floor of the vessel in which it is exposed to the dust, instead of being raised above 
2 c 
VOL. CCI.—A. 
