432 PROFESSOR J. W. MALLET ON THE ATOMIC WEIGHT OF GOLD. 
times redistilled in vacuo, rejecting each time about one-third of the quantity treated. 
The process is easily carried out, and in the final product, completely soluble in dilute 
sulphuric acid without visible residue, no trace of detectable impurity could be found. 
For the evolution of hydrogen on solution of this zinc in acid the little piece of 
apparatus represented in fig. 8 was used, the same that I had used in my work of 
several years ago on the atomic weight of aluminum.* The description formerly 
given of the details of an experiment with this apparatus may he repeated with but 
trilling change of language. A rather more than sufficient quantity of diluted 
Fig. 8. 
sulphuric acid, its volume accurately measured, having been introduced into the bulb 
a by means of a little tube-funnel passed through the tube h, the outer end of 
which was originally open, taking care to leave the surface of h clean, the metallic 
zinc, in a single piece of elongated shape, and having a little bit of slender platinum 
wire wrapped round it, was passed into h, held, nearly horizontal, so that the metal 
did not slip down into the bulb, but rested 40 or 50 mm. from it; h was now drawn 
off in the lamp flame, and sealed with a well-rounded end. The bulb was touched for 
a moment or two with the hand, so as to expel a very little am, and the outer end of 
the small tube c was introduced into the mercury of the trough, taking care that h 
was still kept in such a position as to prevent the zinc coming in contact with the 
dilute acid. After a sufficient lapse of time for the apparatus to have acquired the 
temperature of the room, the barometer and thermometer and the difference of level 
of the mercury m the trough and in c were read off ;t so that, knowing the volume 
of dilute acid introduced and of metallic zinc (the latter from its weight), calibration 
of the bulb and tubes after the experiment was over completed the data necessary to 
determine the volume of air which the apparatus contained at the beginning. The 
* ‘ Phil. Trans.,’ 1880, p. 1026. 
t All readings were, of course, made from a distance with the aid of a small telescope. 
