I 
THICKNESS AND ELECTRICAL RESISTANCE OF THIN LIQUID FILMS. 519 
such precaution was taken, which is certainly opposed to the theory that CO 2 is the 
efficient cause. 
Proposition VIII.—Tlie change in specific conductivity is practically unaltered ifi the 
I films are formed in cm atmosphere of oxygen. 
\ 
When the films were formed in an atmosphere of either oxygen or nitrogen they 
did Dot thin in the normal way. The black was either not formed at all, or, if formed, 
' was streaked wfith lines and flecks of other colours. An electrical measurement 
* therefore only gave the mean apparent thickness of a non-uniform mass, from which 
! the thickness of the black could not be deduced. When the film case was filled, and 
I the film itself blown with purified oxygen, from which all traces of chlorine had been ' 
I carefully removed, it was, however, possible to make observations when the thickness 
! corresponded to the white of the first order. This colour changes so slowly as the 
film thins that no accurate estimate of the thickness can be based upon it. Alternate 
series of measurements were, however, made when the box was filled with air and 
oxygen respectively with the following results :— 
Table XII. 
Air pnrified from CO 2 . 
Oxygen. 
Air purified from CO^. 
Colour of the first 
order. 
Electrical 
thickness 
in 
Colour of the fii’st 
order. 
Electrical 
thickness 
in /i./n 
Colour of the first 
order. 
Electrical 
thickness 
in 
Yellow. 
407 
Yellowish-white . 
403 
White .... 
465 to 383 
White. 
.387 
Grey to white 
326 
White, full of black 
346 
Grey (with black 
296 
Grey to white . . 
316 
spots 
specks) to grey 
Grey. 
286 
Grey (mixed with 
245 
Grey to white . . 
296 
9 ? . 
261 
black spots) to 
9 ) ..... 
236 
white 
It must be clearly understood that the estimates of colours of films as thin as these 
are very untrustworthy, but they are, we think, sufficient to prove that the phenomena 
are substantially identical in air and in oxygen. 
The thickness corresponding to the white of the first order, as given in Newton’s 
table, is 97 /x.^a. Hence, in all cases, the electrical very much exceed the true thick¬ 
nesses. The first two numbers in the case of oxygen (in Table XII.) correspond to 
colours given by thicknesses a little greater and less respectively than that which 
corresponds to the white. Taking the mean of these, and also the mean of the 
extreme thicknesses for which the film appeared white in the second air experiment, 
we get the following Table :— 
