250 ELEMENTARY LESSONS ON [CHAP. iv. 



stantaneous ; it is utterly impossible to determine at 

 which end the flash begins. 1 



297. Electric Dust-figures. Electricity may creep 

 slowly over the surface of bad conductors. Lichtenberg 

 devised an ingenious way of investigating the distribution 

 of electricity by means of certain dust -figures. The 

 experiment is very easy. Take a charged Leyden jar 

 and write with the knob of it upon a cake of shellac 

 or a dry sheet of glass. Then sift, through a bit of 



Fig. 109. 



muslin, over the cake of shellac a mixture of powdered 

 red lead and sulphur (vermilion and lycopodium powder 

 answer equally well). The powders in this process rub 

 against one another, the red lead becoming +, the 

 sulphur - . Hence the sulphur will be attracted to 

 those parts where there is + electrification on the disc, 

 and settles down in curious branching yellow streaks like 



1 Sometimes the flash seems to strike downwards from the clouds, some- 

 times upwards from the earth. This is an optical illusion, resulting from the 

 unequal sensitiveness to light of different portions of the retina of the eye. 



