72 



Prof. J. W. Mallet. 



[May 9, 



cautions are described in detail which were resorted to in purifying 

 the metallic gold to be employed in the research, with the history of 

 some samples of " proof " and " trial plate " gold, obtained from the 

 Mint establishments of the United States and England. The general 

 precautions observed in the course of the work are described, particu- 

 larly the use of an arrangement for evaporating some of the gold 

 solutions with exclusion of the organic matter of atmospheric dust. 



A detailed account is given of the methods adopted in seven series 

 of experiments, looking to more or less independent determinations of 

 the atomic weight sought, viz. : — 



First Series. — Division of a uniform neutral solution in water of 

 auric chloride into two accurately weighed and nearly equal portions ; 

 precipitation from the one portion by sulphur dioxide of metallic 

 gold, which was collected and weighed ; determination, with special 

 precautions, of the quantity of metallic silver required, as nitrate, to 

 precipitate the chlorine in the other portion. 



Second Series. — Similar treatment of a neutral solution of auric 

 bromide, giving the quantity of gold in one portion, and the quantity 

 of silver required to precipitate bromine in the other portion. 



Third Series. — Similar treatment of a solution of repeatedly crystal- 

 lised potassium auri-bromide, again giving the quantity of gold in one 

 portion, and the silver equivalent to the whole of the bromine in the 

 other portion. 



Fourth Series. — Determination of the loss by ignition of a weighed 

 quantity of trimethyl-ammonium auri-chloride. 



Fifth Series. — Comparison of the weights of gold and silver simul- 

 taneously deposited by the same electric current from aqueous solu- 

 tions of auro- cyanide and argento-cyanide of potassium respectively. 



Sixth Series. — -Comparison of the weight of gold deposited from a 

 solution of potassium auro-cyanide on electrolysis with the volume of 

 hydrogen liberated by the same current from dilute sulphuric acid, 

 the hydrogen evolved in a voltameter of special construction, and 

 measured of course under well-defined conditions of temperature and 

 pressure. 



Seventh Series. — Determination of the volume of hydrogen, under 

 known conditions of temperature and pressure, obtainable by solution 

 in dilute sulphuric acid of a given quantity of specially purified 

 metallic zinc ; use of a definite quantity of the same zinc, taken in 

 small excess, to precipitate gold from a neutral solution of auric 

 chloride, and determination of the quantity of metallic gold thrown 

 down ; determination of the volume of hydrogen obtainable on solu- 

 tion in dilute sulphuric acid of the excess of zinc thus used. Resulting 

 comparison of the quantity of gold in solution as auric chloride with 

 the quantity of hydrogen equivalent to the metal. 



The results obtained are then stated as follows : — 



