Equivalents of Oxygen and Hydrogen, 621 
too large and sometimes too small, this possibility may be 
disregarded. Altogether, reviewing the electrical measure- 
ments — E.M.F., resistance, time, and current balance, one 
seems justified in considering that the quantity of electricity 
flowing is known to within one ten-thousandth part. 
4. Volume. 
The mercury displaced by the gas was weighed on a large 
Oertling balance, capable o£ carrying five kilos and sensitive 
to a milligram. The weighing was only carried out to a 
decigram, however, as this is ample for the purpose, some 
2500 grams of mercury being used in an experiment. The 
arms of the balance were found to be sensibly equal : the 
weights consisted of a 2000 g. and 500 g. in brass from a 
rough set, and 200 g. downwards from an ordinary box of 
analytical weights ; the relative errors — amounting to 2 or 
3 decigrams, in the case of the largest — were allowed for, the 
analytical weights being taken as standard. The relative 
accuracy of the weighing may be put at 5QQ(J0 therefore : 
but as to the absolute values I am unable to speak, there 
being no standard in this country. It is, however, hardly 
likely that the errors of the 200 g. and 100 g. in a box of 
weights of first quality would be large enough to matter. 
The volume of the mercury was calculated from the table 
in Landolt & Bornstein, p. 42 : temperature taken being 
the mean between the initial and final temperatures of the 
water-jacket. In favourable cases this only varied one or 
two tenths of a degree during the experiment, so that the 
error in the density of mercury would be of the order of 
6(J Q0Q . The accuracy with which the amount of mercury 
flowing out could be adjusted was less than this, a single 
drop making a difference of one, ten, or twenty thousandth ; 
but it was sufficient. 
5. Pressure. 
The barometer used was one of special design made for 
the author by Messrs. Baird & Tatlock, and filled in the 
laboratory. The tubes, of 25 mm. bore, are large enough to 
eliminate capillary effects; the upper tube carries a glass 
pointer to which the mercury was adjusted * by means of an 
inlet and outlet tap, and a screw-clip compressing a short 
piece of rubber tubing. The mercury used was taken direct 
from the bottle in which it was supplied, and cleaned with 
* The barometer varies very little all the year round at Johannesburg. 
