THICKNESS AND ELECTEICAL RESISTANCE OF THIN LIQUID FILMS. 523 
In the first place it will be noticed that in all cases the ratio is much less than 
when no salt is added to the solution. At optical thicknesses above 450 /n./x., the 
number varies on each side of unity, giving no evidence of a change in specific con¬ 
ductivity. This is proved by the films numbered 2, on July 11 and 12. At thicknesses 
between 200 and 450 /x./x., the ratio is generally above unity, being in some cases as 
large as 1‘27 or 1'28. It is, however, to be observed that there is no clear indication 
of an increase in the ratio as the film thins. The value corresponding to the smallest 
optical thickness is not the greatest in the cases of film 2, July 12, and film 4, 
July 11, and is actually the least of those furnished by measurements on film 3, 
July 11. 
The optical and electrical measurements of the thickness of black films formed of 
a 3 per cent, solution are in as close accord as the nature of the experiments will 
permit, the electrical being as a matter of fact rather the snialler of the two. Thus, 
from Tables VII. and L, we get 10'6 /x./x.,and 12'4 /x./x., for the mean electrical and optical 
thicknesses respectively. The discrepancies between the individual experiments are 
too great to enable us to insist on the accuracy of this difference, but it is evident 
that there is no such striking divergence between the results of the two methods as 
is remarked when no salt is present. 
We have tried from time to time to obtain some measures of the electrical thick¬ 
ness of films formed of a solution containing 3 per cent. KNOg, at thicknesses inter¬ 
mediate between that corresponding to the black and 200 /x./x. Such observations 
would he very valuable in helping us to decide whether there is or is not a real 
maximum of specific conductivity at about 100 /x./x., which is suggested by the fact 
that (in spite of the discrepancies above noted) the ratio of the electrical to the 
optical thickness is less for thick films and for black films than for those whose 
thicknesses lie between 200 /x./x., and 400 /x./x. Unfortunately, though we have 
exhausted every device known to us, sometimes leaving the film to thin unaided, 
sometimes passing weak and sometimes strong currents through it, we have never 
succeeded in getting the white of the first order. This connecting link cannot, there¬ 
fore, be at present supplied, but the facts are sufficient to justify the proposition that 
the increase in specific conductivity observed in the case of an unsalted solution is 
largely diminished by the addition of a salt. It is indeed open to question whether 
the evidence would not support the statement that there is no appreciable change in the 
specific conductivity of a film containing 3 per cent, of salt, but although we hardly feel 
justified in drawing this general conclusion, there can be no doubt whatever that 
it is true for a liquide glycerique for the black on the one hand, and for thicknesses 
greater than 374 /x./x. on the other, and therefore, in all probability (though not quite 
certainly) for intermediate thicknesses. This conclusion rests upon our previous 
work. In 1881 we proved that there is no change in the specific resistance of a film 
of salted liquide glycerique down to a thickness of 374 /x./x. (‘ Phil. Trans.,’ Part 2, 
1881, p. 447, et seq.) while the optical and electrical thicknesses of black films are 10‘7 
3x2 
