356 Proceedings of Royal Society of Edinburgh. [sess. 
The method adopted for the determination of the resistance X 1? 
at the temperature / 15 has already been described in the preceding 
section ; the determination of the two other resistances was pro- 
ceeded with immediately after the completion of the first, the 
same method and precautions being observed throughout. The 
temperature of the tank water was in each case suitably increased 
by the introduction of steam from a boiler. 
The constants a and in the formula 
X, = X 0 { 1 + at + 
were then calculated in the usual manner. 
Determination of the Relative Densities. 
These were determined in the manner already described in a 
former communication,* but with the following additional pre- 
cautions : — 
1. After the pyknometer had been cleaned with hot and fuming 
nitric acid, it was repeatedly washed out with newly redistilled 
water only ; because, according to my own experience, it is far 
•easier to completely remove water vapour than either alcohol or 
ether vapour. 
2. The air used for sweeping out the water vapour from the 
heated pyknometer was well dried and filtered by passing it 
through (1) a large tube filled with calcium chloride, and (2) a 
tube plugged with the finest cotton-wool, through which a strong 
current of air had previously been forced in order to remove any 
loose filaments, which might otherwise easily find their way into 
the pyknometer and so cause 'trouble and inaccuracy in weighing. 
3. In calculating the relative densities given under d 24 in Tables 
II. and III., the usual corrections for reducing the brass weights to 
vacuo were applied, due allowance being made at the same time 
for the weight of the volume (49 c.c.) of dry air which filled the 
generally so-called c empty ’ pyknometer when the instrument was 
first weighed. 
The values enumerated under B in Table I. were obtained by 
reducing the ordinary weighings in the usual manner. 
4. To eliminate the corrections due for the buoyancy of the air 
* Proc. Hoy. Soc . EMn . , 1900, p. 36. 
