400 
PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OF GOLD. 
Ills results. As pointed out by Thorpe and Laurie'"' this partial decomposition of 
the salt was probably due to the action of dust from the air. If the results obtained 
from the solution were used, wdthout any correction, to establish the atomic weight of 
gold, the tendency would of course be to a value lower than the truth. Although the 
correction introduced is small, it can hardly be supposed that it should be taken as 
constant in amount in all the experiments. 
4. In series e it may be questioned whether traces of potassium bromide may not 
have been volatilized at the highest temperature used, or the residual potassium 
bromide may not have, to a small extent, exchanged bromine for oxygen while heated 
in air (before the use of the stream of hydrogen), the latter change being one to be 
guarded against whenever haloid salts are strongly heated in the presence of free 
oxygen. The tendency in both cases would be to a lower atomic weight for gold. 
5. In series f there was risk of slight loss of potassium bromide during filtration 
and evaporation of its solution, and during exposure of the salt to the heat of a free 
flame, wdien there might possibly have been again slight replacement of bromine by 
oxygen, fhus causing the atomic weight sought to come out too high, or else, on the 
other hand, risk of imperfect drying, which would give too low a value for the atomic 
weight^ in question. 
On the whole it seems probable that the tendency of most of the constant errors to 
be suspected in connection with Kruss’s experiments —experiments carried out with 
remarkable patience, skill, and apparent freedom from merely “fortuitous” errors— 
wns in the direction of an atomic weight for gold somewhat below, rather than above, 
the true value. 
B. Experiments oj and Laurie, 1887.t—In these experiments’potassium 
auri-bromide was used, and determinations were made :— a. Of the wmight of the 
residue left on heating the salt over a Bunsen flame till bromine ceased to be given 
off (this residue consisting of metallic gold and potassium bromide), and the weight of 
the gold left by such residue after all potassium bromide had been w^ashed out of it 
by water; 6. Of the weight of silver necessary to be added as nitrate to the solution 
of potassium bromide obtained in a in order to just precipitate the bromine present; 
e. Of the weight of the silver bromide so precipitated. All suitable experimental pre¬ 
cautions seem to have been taken, and the weighings were corrected for atmospheric 
buoyancy. The individual results in each series agreed with each other even more 
closely than in Kruss's research. 
The results obtained were as follows, using in calculation the numbers Ag = 107'G6, 
Br =7975, and K = 39-03 :~ 
Series a. Mean of 8 experiments. Atomic weight of gold = 196-876 
M h. ,, 9 ,, ,, ,, ,, = 196-837 
„ =, 8 „ „ „ „ = 196-84-2 
* ‘ Obem, Soc. Joui-n.,’ Dec., 1887, p. 866. 
t ‘ Chem. Soc. Jom-n.,’ June, 1887, p. 565, and Dec., 1887, p. 866. 
