12 THE ATOMIC WEIGHTS OF 



again from a lead still provided with a platinum condenser. The 

 hydrofluoric acid thus obtained was redistilled four times from 

 an all-platinum still, that is to say, twice with a little pure silver 

 carbonate, and finally twice alone. The product left no residue 

 upon evaporation and was free from perceptible traces of silica, 

 lead, iron, chlorine, and organic matter the chief impurities to 

 be met with in this acid. 



The purification of other reagents, besides those already 

 described, will be given as such reagents may be called for in the 

 prosecution of this study. 



THE APPARATUS. 



For reasons which will become apparent in due time, the final 

 conversions of borax into other salts had to be carried out in a 

 flask or long-necked bulb through which a current of air was 

 passed. Platinum proved to be the only material suitable for 

 this purpose. The platinum bulb applicable to all essential 

 exigencies of the problem, without being too cumbersome or too 

 complicated, is shown in figure 4; the sketch is largely self- 

 explanatory. 



The bulb proper was 4 cm. in diameter and had a capacity of 

 about 30 c.c. The neck was 1.5 cm. wide and the volume of the 

 entire flask approximately 40 c.c. The inner tube, which served 

 to send a current of air through the bulb, was fused near its 

 upper end into a cup-shaped stopper ground into the neck of the 

 flask and, furthermore, was provided with two annular, perforated 

 platinum disks (as indicated in the sketch) which were 4.5 cm. 

 apart and all but touched the walls of the neck. These disks 

 served a threefold purpose. In the first place they acted as guides 

 for the inner tube and thus prevented undue strain upon the 

 ground-in stopper and insured its proper insertion. This function 

 of the disks must not be scorned, for a ground-in, rather shallow 

 platinum stopper of this description although the neck be 

 reinforced, as in the present case is easily rendered useless and 

 becomes a constant source of annoyance unless the two ground 

 surfaces be in proper alignment just before closing. With the 

 present device the stopper easily slid into its seat when the appa- 

 ratus was removed from its supports. In the second place these 

 perforated disks acted as effective "spray-guards" in preventing 



