184 LORD RAYLETGH AND MRS. SIDGWICK 
We found some reason for thinking that the resistance tended to diminish with time 
when the mercury remained long in the tube. To examine this we filled tube II. on 
April 3rd, and found its resistance to be ’99077. It was then left standing full of 
mercury till April 18th, when the resistance was '99055. This difference can hardly 
be relied upon; and in any case the experiments we have tabulated cannot well be 
affected by any change of this kind, as the interval between the measurement of 
resistance and that of volume was very short, except in cases 1 and 7. In case 7 the 
tube stood full of mercury for two days after the resistance was taken. In case 1 the 
resistance was measured on two successive days, and the mean of the two values 
taken. The second was the lowest by '00020, possibly owing to an error. The 
length was measured immediately after the last measurement of resistance. 
The variations in the values of r are, as we should expect, greater than those in R, 
being affected by probable errors in the other data. The extreme difference amounts 
to less than 6 in 10,000, and the greatest divergence from the mean value is 3'3 in 
10 , 000 . 
The mean value of r according to these experiments, *95418, lies between that 
deduced from Dr. Siemens’ experiments for his 1864 standard, namely, '9534, and 
Dr. Matthiessen’s value, namely, -9619 (Phil. Mag., May, 1865), but the difference 
between our value and Dr. Matthiessen’s, namely, '00772, is nearly ten times as 
great as that between ours and Dr. Siemens’. We are unable to account satisfactorily 
for this large difference. One point, however, is worth noting. Dr. Matthiessen 
measured the resistance of the mercury in his tubes, not at zero, but at temperatures 
between 18° and 19°'l (Report of British Association Committee for 1864). To 
deduce the specific resistance at zero, therefore, he must have assumed the coefficient 
of variation with temperature, and presumably —though it is nowhere stated in the 
Report—he used that found from his own experiments (Phil. Trans., 1862), namely, 
'074'’“ per cent, per degree. Our own observations have led us to suspect that this 
value is too small. We made three comparisons of the resistance of tube III. in ice, 
and in water at approximately the temperature of the room, and one similar com¬ 
parison with tube IV. The results are given in the following table. Our arrange¬ 
ments were not adapted for observing the resistance at other temperatures, as the 
open trough afforded no means of checking rapid change. 
* This is the value which results from the experiments made at 0° and at about 20°. 
