506 
PROFESSORS A. W. REIXOLD AXD A. W. RUCKER OX THE 
position of the film and the thickness of the black which it produces. We therefore 
requested that the publication of our paper might be delayed, and we now include 
the results then obtained in an account of further experiments which have been 
carried out as opportunity offered during the last three years. 
We think that it will be most convenient to state our results in a series of pro¬ 
positions, and to describe in each case the experimental evidence on which the 
conclusions are based. It is necessary to state, by way of preface, that the optical 
and electrical methods of measuring the thickness of the films are in all essentials 
identical with those described in our paper “ On the Limiting Thickness of Liquid 
Films,” already referred to, and that the precautions therein detailed have been 
carefully attended to. 
Proposition I. —In the case of soap flms formed ivith a solution of oleate of soda [one 
part of oleate in about 70 of water) containing about 3 percent, of potassium 
nitrate, the mean thickness when they are thin enough to appear black, is the same 
whether measured optically or electrically, and is about 12‘1 p./x. 
The proof of this ^proposition is contained in our paper on the limiting thickness of 
liquid films already referred to. 
It will, perhaps, be remembered that a similar statement is also true when the 
solution is mixed with two-thirds of its volume of Price’s glycerine. 
In the next two propositions we deal exclusively with results obtained by the 
optical method of measuring the thickness of the films. 
Proposition II.—If the proportion of potassium nitrate mingled ivith the solution he 
diminished the thickness of the film as measured by the optical method increases. 
This is proved by the following Table. In all cases oleate of soda was used, the 
proportion being 1 part of soap to 50 of water in the case of the first two liquids, and 
1 of soap to 40 of water in the last two. These variations in the proportion of soap 
to water were made from time to time in consequence of the great difiiculty of 
getting black films at all. Often an entire day was devoted to the work without a 
single result being obtained. It thus became necessary to try a new solution, which 
w^as purposely made a little different from the old one, in the hope of securing some 
black films on which measurements could be made. Even had we been aw^are, at the 
time when many of these experiments were carried out, of the effect on the thickness 
of the film of a change in the proportion of soap, we could hardly have made all the 
experiments under precisely similar conditions. The question as to why a particular 
solution sometimes does and sometimes does not readily thin to the black cannot at 
present be answered, and it is only by varying the conditions that any results can be 
obtained. 
