PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OF GOLD. 
433 
piece of zinc was now made to slide down into the bulb, tlie end of the gas delivery- 
tube c liaving been brought under the mouth of the measuring flask. Over-rapid 
evolution of hydrogen and any considerable rise of temperature were prevented, partly 
by tilting the bulb so that the little piece of zinc rested against one side and exposed 
but a part of its surface to the action of the liquid, and partly by cooling the outside 
of the bulb with water. To guard against more than traces of aqueous vapour 
being carried away with the hydrogen, a rapid current of ice-water was kept up 
throuo-h d. 
As soon as the last of the zinc had disappeared, leaving the liquid quite clear, c 
was brought up into a nearly vertical position, and the apparatus left to itself until 
the temperature of the room had been attained. The barometer and thermometer and 
the height of the mercury in c above that in the trough were now read and recorded. 
Lifting c straight up from the trough, the mercury in this tube was got out by 
running a wire up and down in it, and, inverting it, the whole of the remaining space 
in a, b, and c was filled up with solution of zinc sulphate and free acid of the 
same strength with that already contained, this liquid being run in from a graduated 
burette through a slender tube-funnel, and the volume used noted, so as to show how 
much liquid had been already present. 
Idle apparatus having been now emptied, washed out, and calibrated (with water, 
instead of mercury, on account of the difficulty of getting the interior quite dry), the 
volume of gas remaining in it at the close of the experiment was had from the 
difierence between the total capacity (to the level of the mercury in c) and the 
volume of liquid which the bulb had contained at the close of the experiment, these 
taken together with the data for pressure and temperature. 
The dilute acid was saturated with pure hydrogen just before being used (and in 
the experiments with auric chloride or bromide the main portion of water holding this 
salt in solution was similarly treated), and a preliminary experiment showed that there 
was but an extremely minute difference between the amount of gas removable from 
such liquid by heating in a Sprengel vacuum and from that containing zinc sulphate 
after the solution of the metal ; so that, practically, the question of retention of gas 
in solution by the liquid might be neglected. 
The sulphuric acid was diluted to 25 per cent, by weight, only a small bit of 
platinum wire was wrapped round the zinc, and the temperature of the bulb was not 
allowed to rise beyond about 20° C. Thus the risk of evolving other gaseous 
products than hydrogen—as hydrogen sulphide or sulphur dioxide—was avoided, 
and on testing for these impurities the hydrogen collected no traces of them were 
found. 
The measuring flask used to collect the hydrogen was of the same character as 
that used for the experiments of the sixth series, but of much larger size, holding 
* Muie and Adie : “ On tlie Interaction of Zinc and Sulphiii’ic Acid,” ‘ Cliem. Soc. Journ.,’ Jan., 
1888, p. 47. 
MDCCCLXXXIX.-A. 
3 K 
