PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OF GOLD. 
411 
warm water, metallic gold only remained imdissolved, which was then filtered off. 
This neutral solution having been rendered uniform by agitation, two approximately 
equal portions of it were weighed off, using, of course, stoppered vessels to prevent 
evaporation during the weighing. From one of these portions the gold was thrown 
down in the metallic state by pure sulphurous acid with the aid of heat, carefully 
collected, well washed, dried, ignited in a Sprengel vacuum, and weighed. To the 
other portion there was added the carefully prepared solution in a minimum of nitric 
acid of an accurately weighed quantity of pure silver, a little more than equivalent to 
the chlorine present, the liquid and precipitate digested together for a considerable 
time with gentle warming in a stoppered glass flask, well agitated from time to time, 
and the precipitate (of silver chloride, containing also the gold) filtered off upon 
siliceous sand, and thoroughly washed, avoiding throughout the decomposing influence 
of light. The clear filtrate was nearly neutralized with pure sodium hydroxide (from 
metallic sodium), evaporated down to a small bulk, using the vessel represented in 
fig. 2 (p. 410), and finally the remaining silver was determined (with all the needful 
precautions of the silver assay) by means of a weigfied quantity of a weak solution of pure 
hydrobromic acid standardized against pure silver. This mode of determining chlorine 
by means of silver and hydrobromic acid was suggested to me in a letter, of the 
27th January, 1887, with which I was favoured by M. Stas,'" who advocates it as 
the most exact process available, d'he pure silver required was prepared in the same 
way as that used in my experiments on the atomic weight of aluminum,t and was 
heated in the Sprengel vacuum to remove all occluded gas. The hydrobromic acid 
was prepared as directed by Stas in his published paper—“ De la determination du 
rapport proportioiinel entre I’argent, les chlorures et les bromures.”J 
In reporting the results obtained, the quantity of gold stated is that actually 
weighed, but the quantity of silver corresponding thereto has, for the sake of 
simplicity, been given as that required for an exactly equal quantity of the auric 
chloride solution, while, as stated above, the quantity of liquid weighed off was very 
nearly, but not exactly, equal to that from which the gold was thrown down, the 
difference being allowed for in calculation. 
With this explanation the results of the first series of experiments were as 
follows :— 
* In this letter M. Stas says, “ Je me permets de vous recommander I’emploi de I’acide bromhydriqne 
pour la precipitation de I’ai-gent reste dans nn liqnide apres nne double decomposition operee a I’aide d’un 
chlorure et d’un sel d’argent. On reussit a condition que Lean mere renferme nn exces d’argent dont le 
poids est le triple du metal qui pent rester eu solution a I’etat de chlorure d’argent.” 
t ‘Phil. Trans.,’ 1880, p. 1020. 
X ‘ Memoires de I’Acad. Royale des Sciences de Belgique,’ vol. 43, 1882. 
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