of Edinburgh, Session 1883 - 84 . 
425 
removed by it. When the zinc disc was rubbed with sand paper 
and treated similarly, the following series of observations was 
obtained : — 
Eepeated Brushing. Eepeated Brushing. 
+ 10 +23 0 
+ 28 +12 10 
+ 27 0 
+ 15 0 
Mean +19 +19 2-5 
The observations of 17th December, given in full in Table II., 
may be taken as illustrative of the other sets of observations. The 
morning was frosty, the stem insulated without being dried, the 
brush was heated slightly before beginning* The former series was 
taken with the discs as found ; and the latter, after the discs had 
been polished with emery paper and cleaned with flannel. The 
order of entry is the order in which the observations were made. 
It will be observed that the initial and final readings for zinc in the 
first series are nearly equal, showing that the state of the brush was 
practically constant throughout. In the case of the second series, 
four readings were first taken, then the disc was whisked with the 
brush backwards and forwards across the middle zone about a dozen 
times, and then six more readings were taken. The order of the 
metals is the same, whether the former or latter mean of the second 
series is taken — zinc, lead, iron, copper, tin, — and this order agrees 
with that deduced from the entire collection of observations, except- 
ing that lead is more negative than iron. 
The comparisons entered for 18th December were obtained 
without rubbing the discs previously. The order is the same as 
the latter order of the previous days, excepting that tin comes 
out less than copper, which I believe to be erroneous. On the 
19th the discs were previously rubbed with chamois leather. Zinc 
and lead were at first positive. On these two days an experi- 
ment was made with the zinc disc. On the former day it was 
rubbed with emery paper and chamois leather, and the mean of the 
first four readings was +111. By repeated whisking it was brought 
down to zero, and changed to negative. The disc was again rubbed 
with emery paper and chamois leather, tested once, and found to 
