196 
DE. W. .T. RUSSELL ON THE FORMATION OF 
plate, and lig. 29 Is the effect produced Avheii 8 millims. away ; but when the pin was 
10 uulliius. away, uo effect was produced. If the pin does not touch the plate, as it 
did ill the foriiier case, its height again does not affect the deposit formed. If the 
pill he placed at a lower level than the plate, and at different distances from it, it is 
still able to produce a deposit on the plate, as was proved by trying it at a constant 
distance of ll, millim. below the level of the jffate, and at distances of 2, 4, 6, 8, and 
10 millims. from the plate. At 2 millims. a consideralile amount of deposit was 
formed, and the amount gradually diminishes and recedes from the edge of the plate 
as the distance increases. At 2 millims. from the plate, the deposit is nearly up to 
the edge. At 4 millims. it commences at 18 millims. from the edge, at 6 millims. 
at 27 millims., at 8 millims. at 30 millims., and at 10 millims. there is no deposit 
formed. 
It the pin be placed at a still greater depth below the level of the jjlate, it is still 
able to 2 >i'oduce a deposit on the jilate, the deposit, of course, becoming less as the 
Fig- 39a. Fig. 30. 
depth increases. At 4 millims. below the level oi the plate, and 2 millims. away 
from it, a small de2)osit is formed (fig. 29 a), and even when it is 6 millims. beloAv, a 
visible deposit is formed at the centre of the jffate. The amount of de^Josit produced 
l)y a pin on the same level as that of the plate, may be equalled, but apparently is 
nevei exceeded. It has already been shown that no deposit takes place on warming 
and exposing to dust a circular jilate, hut it a pin be placed at different distances 
from it, and either above or below it, deposits are j^roduced similar to those formed on 
any other shaped plate. 
It is certainly remarkable tliat a ^lin so far from the jilate and so much below it 
should he able in so definite a way to aflect what is taking jilace iqion it. 
there still remained another way in Avhich the jiin could he iiresented to the |ilate, 
namely, by holding it above the jilate. If a pin 50 millims. long he held 6 millims. 
above and 3 millims. beyond a plate it produces no effect on the figure, but if the pin 
be sinijily lowered, so that it is only 4 millims. above the [date, then a slight deposit 
