96 Proceedings of Royal Society of Edinburgh. [sess. 
of electrifying it. A little piece of mirror is cleaned and a coin 
placed on its surface. The coin is then heated and left to cool, 
after which it is removed from the glass. If the mirror be now 
breathed on, an image of the coin becomes visible. The coin may 
be heated by placing on it a piece of metal heated to the necessary 
temperature, but it has been found more convenient to heat it by 
means of a gas blow-pipe flame. A blow-pipe, with the air jet in 
the centre of the tube supplying the gas, and mounted so as to 
be capable of being pointed vertically downwards, does very well for 
the purpose, as by means of it the coin can be heated without the 
hot gases from the flame touching the glass. A temperature of 
212° Fahr. gives results, but a higher temperature is better. 
If we take a coin in the condition it is in while in circulation, and 
without cleaning it place it on the mirror and heat it, we shall get 
an image of the coin, which shows quite distinctly without breathing 
on it. The impurities on the surface of the metal are driven off by 
the heat, and form a whitish-looking deposit on the glass, sufficient 
to give a perfectly distinct image, showing most of the details of the 
engraving. The coin should therefore, before being used for pro- 
ducing breath figures, be heated to a higher temperature than it will 
be exposed to afterwards, and great care must be taken that the 
surface of the coin we are going to use is not touched afterwards, 
as it is found that if we clean the coin by heat, and afterwards 
finger it, it will again on being heated give an image visible with- 
out being breathed on. 
The breath figures produced by heat are sometimes very distinct, 
every detail in the coin being reproduced with sufficient clearness to 
admit of the use of a magnifying lens to examine the details. In 
other cases, for reasons difficult to explain, the images are not so 
distinct ; but, from the extreme minuteness of the alterations pro- 
duced on the surface of the glass on which these figures depend, 
this is only what might have been expected. 
Though the deposit of dust from a blow-pipe flame causes the 
condensed vapour to form a uniform film, yet this action of the 
surface of the glass, or rather of the impurities on its surface, may 
be destroyed, and the surface without being cleaned may be made 
to give the ordinary light-scattering form of condensation. If, for 
instance, the plate be highly heated, its surface will afterwards 
