PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OP GOLD. 
429 
the mercury marked a point somewhere within the length of the neck, which point 
was noted by the millimetre scale, the thermometer immersed in the flowing water, 
and the barometer and its attached thermometer being read at the same time. It 
remained only to insert the stopcock stopper under the mercury of the little mercury 
trough, close the stopcock, withdraw the flask from the trough, reject the di’op or two 
of mercury from the stopcock orifice by means of a wire, remove the portion of 
mercury left in the neck of the flask, and weigh it carefully. Its weight, with con¬ 
sideration of its temperature when the stopcock was closed, gave the volume of the 
portion of the flask not occupied by hydrogen, and this, subtracted from the whole 
volume of the interior of the flask, as found by the original calibration, gave the 
volume, under known conditions of temperature and pressure, of the hydrogen which 
had been collected. From two calibrations at different temperatures a correction was 
obtained for the expansion of the glass of the flask, but it \yas hardly necessary to 
take this into account, in view of the small limits within which temperature varied in 
all the experiments made. 
But three experiments carried out by this method led to results which seemed 
worthy of confidence. These results were as follows ;—- 
Experiment. 
Character of gold 
in solution. 
Character of gold 
in plates. 
Gold deposited. 
Hydrogen 
liberated. 
Vol. at 0" C. and 
760 mm. 
Weight. 
grm. 
c.cm. 
grm. 
I. 
A, b 
c 
4'0472 
228-64 
-U20.o3 
II. 
A, b 
c 
4-0'226 
227-03 
•02039 
III. 
A, b 
c 
4-0955 
231-55 
•02079 
In calculating the weight of hydrogen from its observed volume, Begnault’s value 
for the weight of a litre of this gas at 0° C. and 760 mm. was taken as the basis. The 
correction, of which Lord Rayleigh not long since pointed out the need—namely, for the 
compression of the vacuous glass flask by atmospheric pressure—was adopted from the 
experiments of J. M. Crafts (‘ Comptes Rendus,’ vol. 106, p. 1662) ; and his corrected 
value, '08988 grm., was still further corrected for the difference in the force of gravity 
at Paris and at the University of Virginia (in C. G. S. units, 980'94 ; 979'95), giving 
as the value to be used '08979 grm. 
The electrolysis of the water was carried on very slowly, so as to keep the density 
of the current low with such small electrodes as were used. Nevertheless, as the 
hydrogen voltameter required constant watching, it became necessary to bring the 
whole time of an experiment within moderate limits, and hence a considerably 
stronger current was used than in the simultaneous deposition of gold and silver in 
the fifth series, this circumstance being less favourable to the satisfactory deposition of 
the gold. It would have been desirable to use a larger flask, and to collect a greater 
