﻿Dr. T. J. Baker on Breath Figures, 763 



of contamination is removed from those parts of the glass 

 more rapidly than from the neighbouring areas, and a 

 breath figure revealing the design of the coin may be 

 developed. If the time during which the discharge takes 

 place is too prolonged, the whole of the film beneath the 

 coin is removed, all details disappear, and only a disk of 

 " black "condensation is obtained when the glass is breathed 

 upon. 



Electrical conductivity of breath figures. 



The author has found that the passage of a flame or of a 

 stream of electric sparks across the surface of "cleaned" 

 glass greatly reduces the insulating property. 



Two ebonite rods were capped with small pads of tin-foil, 

 one of which was earthed, and the other was connected by a 

 wire to a charged electroscope. By pressing the two pads 

 simultaneously on the surface of the glass to be examined 

 any leakage across the intervening portion of the glass is 

 easily detected. 



Flame tracks produced by burning coal-gas, hydrogen, 

 and carbon monoxide all show considerable conductivity. 

 Coal-gas appears to be most effective, and this is not un- 

 likely, because sulphurous acid is one of the products of its 

 combustion. 



If a flamed plate is heated in a steam oven for about 

 30 minutes the conductivity of the flame track is found to 

 have been reduced very greatly, and a similar result is 

 obtained after a flamed plate has been left in vacuo for a few 

 hours. Transfers are also found to possess a certain small 

 conductivity. 



It is a matter of indifference whether the electroscope has 

 been charged positively or negatively. 



The reduction in conductivity brought about by heating to 

 100°, or by exposure to reduced pressure, suggests that water 

 derived from the burning of the coal-gas or hydrogen may 

 account for the effect, but it is not clear why the flame of 

 carbon monoxide or electric sparks should produce con- 

 ductivity. 



Discussion of results. 



It is probable that a breath figure produced by a flame or 

 by electric sparks is to be attributed in part to the burning 

 off or volatilization of the thin film of contamination which 

 is present on a surface which has been rubbed with "clean" 

 linen. The track of the flame or spark then presents an 



