963 
only the temperature of that bulb is kept below a certain critical 
value, varying for each metal. For mercury he fixed this tempera- 
ture at —135°, for silver he estimates it at 575°. Though the room 
temperature at which the glass was kept during our experiments, 
is far below this critical temperature of the silver, so that we may 
assume that the silver-molecules striking against the bulb do not 
reflect, yet it seems unacceptable, that they resume their position 
of rest immediately after the collision. The glass- and silver-molecules 
situated near the point of impact will be disturbed in their state of 
equilibrium and get into a state of motion; small separate particles 
of silver come into each other’s sphere of attraction and find an 
opportunity of uniting into greater conglomerates. This motion will 
be so much the stronger and therefore the chance of the agglomeration 
of silver-molecules so much the greater, in proportion as the temperature 
of the bulb that is struck, is higher. By maintaining a low 
temperature of the bulb during the entire duration of the burning 
of the lamp, the possibility of acquiring an entirely structureless, 
amorphous deposit is heightened. 
Hereupon some experiments were made in which the silver was 
sublimated, while the lamp had been cooled down to a temperature 
of liquid air, this temperature being maintained during the whole 
time of burning The sublimate showed pretty nearly the same 
colours and succession of colours as those which arose in room 
temperature, any appreciable difference could not be observed. Thus 
a lowering of temperature to below 20° has no material effect on 
the nature of the film, when observed with the naked eye. 
If however, the temperature is raised after the deposit has formed 
a change of colour sets in, in high vacuum as well. By heating 
to 260° for 20 minutes it passed back from reddish-violet to vellow- 
ish-brown, and yellowish-brown to a faint yellowish-green. 
Ultramicroscopically this deposit showed larger particles, being 
better discernible by themselves. Several of them had loosened from 
the bulb and freely moved in the immersion-liquid. 
Farapay') and Berr ®) also experimenting on the much thicker 
deposits of gold and silver which they had prepared by lamination 
of compact metal or precipitation by chemical process, both observed 
a retrogression of the tint and an agglomeration into larger particles 
as the effect of heating. When we remember that as a rule an 
enlargement of the particles is accompanied by a deepening of the 
1) 1. c. page 1. 
2) 1. c. page 2. 
