434 
PROFESSOR J. W. MALLET OR" THE ATOMIC WEIGHT OP GOLD. 
about a litre. The quantity of zinc taken for each experiment was calculated to give 
a volume of gas which, under the conditions of temperature and pressure of the day, 
would bring the mercury to somewhere near the middle of the neck, and the gas, 
previously dried by balls of fused potash, was measured after the temperature had 
been rendered as nearly as possible fixed by the circulation round the outside of the 
flask of-an active stream of water from the laboratory supply pipes. On account of 
slight rise of temperature during the solution of the metal, the volume of hydrogen 
left in the bulb and tubes was always less than that of the air in the same at the 
beginning ; and, after reduction to normal temperature and pressure, the difference had 
to be subtracted from the gas collected in the flask. 
In the experiments with auric chloride or bromide the quantity of hydrogen given 
off on solution of the surplus zinc was so small that it could be easily measured in a 
little gas tube, the same method of double calibration with mercury being used as for 
the larger volumes. In these experiments the bulb used had a second side tube, f, 
as shown in fig. 9, to hold the sulphuric acid, while a contained the aqueous solution 
Fig. 9. 
of the gold salt; this acid was already somewhat diluted, and was introduced into a, 
after complete precipitation of the gold, very gradually, so as to avoid any consider¬ 
able rise of temperature. The quantity of water used was such as to make the whole 
volume of liquid very nearly the same in the experiments with zinc alone and in 
those with zinc and the auric salt. Care was taken to ascertain, after measurement 
of the hydrogen, that it had been efl’ectually freed by the potash balls not only from 
moisture, but from any traces of hydrochloric acid formed and carried over. 
In order to connect the weight of the zinc with that of the hydrogen produced by 
its solution, it was necessary that the weight of the metal should be absolute, or in 
terms of equal value with those used in Regnault’s researches on the density of 
hydrogen; hence, as has been already stated, the weights used were such as had had 
their real values determined, and the precaution of double weighing was applied. 
