482 
PROFESSORS A. W. REIKOLD AND A. W. RUCKER OK 
obtained, and, combined with the extreme constancy of the thermometer and hygro¬ 
meter, proves that a state of things was accidentally produced very similar to and of 
even greater constancy than that attained by the ordinary methods of moistenino-. 
With this single exception, however, all the films included in Class I. obey Ohm’s law 
better than any of the others. 
In order to test this agreement more fully the following method was adopted. The 
simplest plan for combining the results of a set of films would be to take the means of 
the specific resistances at each of the thicknesses at which they were observed. In 
the case of the films in Class I. the range of comparison possible with this method 
would have been only from 14X10“° to 9X10“ 5 centims., the greatest and least 
thicknesses at which all the films were observed. 
To get over this difficulty the specific resistance obtained at each thickness for any 
film was divided by that corresponding to a thickness of 12x10“° centims. for that 
film. If Ohm’s law were not obeyed, each of the columns of figures thus obtained 
ought to give increasing or decreasing values of the ratios as the thickness diminished, 
and this divergence from the law would have been still more strongly marked in the 
means of the numbers corresponding to any particular thickness. 
The results of this calculation are exhibited in the following table. 
Column I. gives the apparent thickness in terms of 10“° centims. 
Column II. the colour corresponding to this thickness correct to the nearest tenth of 
a tint. 
Column III. the actual thickness. 
Columns IV.-IX. the ratios for the six films. 
Column X. the means of these ratios. 
The date and number of the film and the readings of the hygrometer and thermo¬ 
meter at the beginning and end of the observations are placed at the head of the 
column which refers to it. 
The thickness 12 X 10“° centims. was selected as the point of comparison, as at that 
thickness the specific resistances differed least from their mean value. 
