94 
Proceedings of Royal Soeiety of Edinlurgh. [sess. 
Breath Figures. By John Aitken, Esq., F.E.S. 
(Read May 1, 1893.) 
These well-known figures are generally produced by placing a 
coin on one side of a piece of glass, and on the other side opposite 
it another coin or small plate of metal. The coin and the plate 
are then strongly electrified— the one positively, the other nega- 
tively. When the coin is afterwards removed, and the surface of 
the glass on which it rested is breathed on, there is developed an 
image of the coin, showing many of the details of the engraving. 
The image is produced by the condensed moisture being deposited 
in different-sized patches at different places. This is easily seen by 
examining the image by means of a microscope, using a low power, 
and illuminating the image by means of an ordinary mirror, with a 
large black spot fixed in the centre. At the parts wEere the image 
is bright, it will be seen that the moisture is deposited in very 
small detached patches of nearly equal size. The surface looks as if 
it were covered with a layer of small plano-convex lenses of quick 
curvature, with their edges close to each other. Where the image 
is darker the lenses are larger and flatter. As the amount of vapour 
deposited is about equal at all parts, the smaller the lenses the 
quicker will be their curvature ; and it is the light scattered by the 
great number of small reflecting surfaces which causes one part of 
the image to be brighter than the other. 
It seemed possible that in some cases these breath figures might 
depend in some way on the dust or other impurities on the surface 
of the glass, either deposited on or in some way altered by the 
electrical conditions to which it was subjected. If so, then it 
appeared to me that possibly heat might produce similar results 
by the molecular bombardment to which the surface of the cold 
glass would be exposed by the gases heated by the coin. When 
tried, heat wns found to give breath figures similar to those pro- 
duced by electrification. 
The effect of the very fine dust from a flame when deposited 
on the glass was also tested ; and as this experiment illustrates the 
