1892 - 93 .] 
Mr Aitkeii on Breath Figures, 
95 
manner in which the bright and dark parts are produced, which give 
the appearance of shading to the images, it will be described first. 
Clean a glass plate. Small pieces of mirror are the best for these 
experiments, as the images shine out more brightly on mirrors than 
on ordinary glass. The mirror should be polished till it shows an 
equal surface all over when breathed on. It may be remarked here 
that the ordinary process of testing the cleanness of a glass surface 
by breathing on it is simply a practical application of breath figures. 
Having got the surface free from figures, it is in a condition for 
receiving impressions. If we pass over this clean surface the point 
of a blow-pipe flame, using a very small jet, and passing it over 
the glass with sufficient quickness to prevent the sudden heating 
breaking it ; and if we now breathe on the glass after it is cold, 
we shall find the track of the flame clearly marked. While most of 
the surface looks white by the light reflected by the deposited 
moisture, the track of the flame is quite black ; not a ray of light is 
scattered by it. It looks as if there were no moisture condensed on 
that part of the plate, as it seems unchanged ; but if it be closely 
examined by means of a lens, it will be seen to be quite wet. But 
the water is so evenly distributed, that it forms a thin film, in 
which, with proper lighting and the aid of a lens, a display of 
interference colours may be seen as the film dries and thins away. 
Another way of studying the change produced on the surface of 
the glass by the action of the flame is to take the mirror, as above 
described, after a line has been drawn over it with the blow-pipe 
jet, and when cold let a drop of water fall on any part of it where it 
showed white when breathed on. Now tilt the plate to make the 
drop flow, and note the resistance to its flow, and how it draws 
itself up in the rear, leaving the plate dry. When, however, the 
moving drop comes to the part acted on by the flame, all resistance 
to flow ceases, and the drop rapidly spreads itself over the whole 
track, and shows a decided disinclination to leave it. This experi- 
ment shows why one part of the plate wffien breathed on is white 
while the other part is black. Over one part the condensed vapour 
draws itself into little lenticular patches, while over the other it 
spreads itself in a thin film, having no power to scatter light. 
To produce breath figures by means of heat, similar to those pro- 
duced by electricity, all that is necessary is to heat the coin in place 
