480 
PROFESSORS A. AT. REEXOLD AXD A. W. RtiCKER OX 
sufficiently to produce an appreciable effect upon the specific resistance, might never¬ 
theless have reached a stable state during;' the first few minutes after its formation and 
while still too thick to admit of any optical observations being made upon it. If, then, 
the films, the specific resistances of which agreed most closely with that of the liquid 
in mass, were also those which obeyed Ohm’s law with the greatest accuracy, this 
result would evidently point to the conclusion that the variations in specific resistance 
were due to changes in constitution and not to the diminishing thickness of the film. 
To put this to the test the specific resistances of the films were calculated for 
apparent thicknesses which were multiples of ten millionths of a centimetre above and 
for multiples of five millionths of a centimetre below 9 X 10~° centims. The results, 
which are too long to be given in full, will be sufficiently indicated by the following- 
table. The films are divided into four classes. Those in Class I. satisfy the three 
conditions as to constancy of hygrometer, constancy of temperature, and height of 
hygrometer. Those in Class II. satisfy the first two of these conditions, but not the 
third. Those in Class III. fail to satisfy the third and one at least of the other 
conditions. In the case of the films included in Class IY. the arrangements as to 
moistening the air differed in some respects from those above described (pp. 462, 478). 
Column I. gives the date on which the film was observed. 
Column II. the number of the film. 
Column III. the amount by which the ratio of the specific resistance at the least to 
that at the greatest observed thickness differs from unity. 
In each class jx 1 is the mean of these numbers without reference to sign, is the 
algebraical mean. One film (September 29, Y.) is omitted from this list. It was 
purposely observed under conditions (to be presently described) such as first to 
increase, and then to diminish the specific resistance, and is therefore useless for the 
purposes of such a comparison 
