276 
IOWA ACADEMY OF SCIENCE 
oxygen was exhausted, I now turned to the use of mica flakes for insula- 
tion material. First when a continuous sheet of mica of 25 wave lengths 
separated the flats the insulation was imperfect. If charged to a poten- 
tial of 4 volts, three-fourths of the charge would leak off in 10 seconds, 
but by thoroughly heating the plates and the mica over' an electric oven 
the insulation became quite perfect. This was more than a month after 
my experiments with the iron surfaces, and I now found it quite impos- 
sible to obtain good insulation without heating the plates and the insu- 
lator. Presumably the excess humidity was the trouble. 
In one instance the speculum surfaces were separated by three mica 
flakes of total area about 5 mm 2 . By the microscope one was about 14 
wave lengths, the second 12, and the third was lost. When the elec- 
trometer was charged to +5 volts and mixed with the speculum con- 
denser the capacity was 310 cm, and when it was charged to — 5 volts 
the capacity was 315 cm. The total pressure on the upper plate was 
about 50 gms. From this capacity was subtracted 59 units as a cor- 
rection due to the added capacity due to the mica flakes and to the 
plate aside from its condenser action. The area, of the plates was 
1.84 cm 2 . The distance between, the surfaces was therefore calculated 
to be .00060 cm. It seems safe to conclude from this, experiment that 
the difference between the electrical and optical methods is not more 
than three wave lengths. 
It was observed that if the pressure on the plates was increased to 
100 gms. that the insulation became very poor. Sometimes, the insula- 
tion would be restored by removal of the pressure and sometimes not. 
However, increased pressure did not always alter or destroy the insula- 
tion. 
Again three very thin mica flakes were selected. By direct reflection 
they all appeared a brilliant green, but when viewed by the light that 
struck at a very large angle of incidence some of them appeared orange 
colored. With these flakes between the flats the speculum, slid over each 
other freely as if nothing was between, and likewise they resisted pulling 
apart. The resistance between the plates was very low, although I did 
not measure it. Continued heating with the two plates together did not 
restore the insulation, although the mica had been heated for hours 
before the flakes were separated off the sheet mica. 
Then the plates were separated and heated for two hours over an 
electric oven. The temperature of the plates was kept at about 215°. 
The mica flakes stuck tight to the plates during this procedure, although 
some of them did break in pieces. By this the insulation was made 
