PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OF GOLD. 
431 
Seventh Series of Experiments. 
[n pursuance of the attempt to connect directly the atomic weight of gold with that 
of hydrogen, metallic zinc was prepared as nearly as possible in a state of purity, 
and, a known quantity of the metal having been dissolved in dilute sulphuric acid, the 
amount of hydrogen evolved was determined by volume. A solution of pure auric 
chloride or bromide was then treated with a known quantity of the same zinc, more 
than sufficient for the complete precipitation of all the gold present; the excess of 
zinc was dissolved by dilute sulphuric acid, and the volume of hydrogen given off was 
determined. The precipitated gold was carefully collected, washed, dried, ignited, 
and weighed. The difference between the volume of hydrogen which the zinc gave 
when thus partly used to replace a known quantity of gold and the volume which it 
would have given if replacing hydrogen alone represented, of course, the volume of a 
quantity of hydrogen equivalent to the gold precipitated and weighed. From this 
volume, under known conditions of temperature and pressure, the weight of the 
hydrogen was calculated on the basis of Regnault’s results for the density of the gas, 
after application of the needful corrections, as in the sixth series of these experiments. 
In a preliminary notice of my work read before the Chemical Section of the British 
Association at the Manchester meeting of 1887, it was pointed out that the method 
just described has certain advantages in principle. It does not require that the 
weight of the gold salt in solution be known, so that all difficulties in regard to drying- 
such salt without decomposition are disposed of. It does not depend upon a know¬ 
ledge of the atomic weight of the halogen in combination with gold, or upon a 
knowledge of the atomic weight of zinc. It does not even require that the zinc be of 
assured purity, provided only it be uniform in character, so that a given weight of it 
can be trusted to yield always the same quantity of hydrogen, and there be no 
impurities present capable of interfering with the collection of the whole of the 
precipitated metallic gold in a state of purity. The chief difficulty cousists in the 
accurate ascertainment of dhe total volume of hydrogen evolved from the solution of a 
satisfactorily large quantity of zinc; when the gold solution comes to be used, as the 
volume of hydrogen given off on solution of the surplus zinc may be made quite 
small, its measurement becomes both easy and exact. 
The pure zinc required was obtained by fractionally distilling in a Sprengel vacuum 
some very nearly pure metal from the Bertha Zinc Works, in South-western Virginia, 
using a long combustion-tube of hard Bohemian glass, and substantially the same 
arrangement of apparatus as that described by Morse and Burton* in connection 
with then- work on the atomic weight of zinc. The original metal was found, by an 
analysis in the laboratory of the University of Virginia, to contain less than '04 per 
cent, of foreign matter, almost solely consisting of lead and iron. It was four or five 
* ‘ American Cliemical Journal,’ vol. 10, p. .312. Tubes of glazed porcelain, closed at one end, had 
been specially procured for use in thus distilling zinc, but it was found that they were quite unnecessary. 
