PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OF GOLD. 
407 
atomic weight determinations. All of tliese fractions received a very thorough final 
washinof with water. 
Such part of the purified metal as was to be used in the })reparation of gold com¬ 
pounds was not fused, but was heated in a glazed porcelain tube to moderate redness 
in a Sprengel vacuum. A small part of the metal used in the free state, and desired 
in compact form, was fused in a perfectly clean 14eaufaye crucible with a little acid 
sulphate of potassium and borax, the button flattened, boiled with strong nitric and 
then strong hydrochloi'ic acid, thoroughly washed with water, and, finally, heated in 
the Sprengel vacuum. Throughout the long process of purification, and especially 
towards its close, the most scrupulous care was taken to exclude dust, and to prevent 
grains of sand from the bottoms of beakers or any other impurities getting into the 
precipitated gold, upon which the acids used would not act, so as to obviate the risk 
of merely mechanical contamination, which, if overlooked, might lead to that being- 
weighed as part of the gold which was, in fact, foreign to it. 
2 . Purification of “Proof Gold” obtained from the United States Mint at Phil¬ 
adelphia .—I owed to the kindness of Mr. J. B. Eckfeldt, Chief As.sayer to the Phil¬ 
adelphia Mint, a liberal supply of the “ proof gold ” used in checking the gold assays 
there made, and he furnished me the following statement of the manner in which this 
purest metal is prepared, under his directions :—“ The best cornets from the gold 
assays selected a,nd dissolved in aqua regia. Solution evaporated, with additions of 
HCl, to nearly crystallization, diluted largely with water, a^d flowed to stand for 
three or four weeks. About seven-eighths of the solution drawn off" from the silver 
chloride, and passed through several thicknesses of various filters. Solution somewhat 
concentrated, and alcohol and potassium chloride added, allowed to stand for some 
time (precip. traces of platinum*), and carefully filtered. Gold precipitated by 
addition of pure ferrous sulphate. Reduced gold washed repeatedly in boiling HCl, 
until washings show no iron, then well washed in pure water. Gold dissolved, and 
solution evaporated to ciystallization, with rejjeated additions of hydrobromic acid,t 
diluted, and again allowed,to stand for some time; filtered. Through the solution 
was passed pure SO., until all the gold was reduced ; washed. Gold again dissolved, 
evaporated with HCl, diluted, and oxalic acid added, and heated, until all gold is 
down. Melted in white clay crucible with potassium chlorate and nitrate, afterwards 
with pure sodium carbonate and borax.” Mr. Eckfeldt also informed me erbally 
that the proof gold thus purified is cast into a small bar in a perfectly clean and bright 
cast-iron mould; the bar is boiled in nitric acid, washed and dried, rolled between fine 
* The platinum of South American native gold, and of scrap gold from dentists, is at the Philadelphia 
Mint separated solely by alloying -svith enough silver, and dissolving out the latter metal with nitric 
acid. The platinum dissolves with the silver. 
t I had, many months before, independently adopted and used hydrobromic acid to remove traces of 
silver more effectually than by hydrochloric acid, when I learned from Mr. Eckfeldt that he had thus 
habitually employed it. 
