THE PREPARATION' OF MATERIALS. 35 



A second sample of silver, prepared from somewhat less pure initial 

 material, was put through a more elaborate and exhaustive round of 

 purification which need not be detailed. The preparation included five 

 crystallizations as nitrate and two successive precipitations as metal by 

 formate. This silver was fused into buttons of a size suitable for imme- 

 diate use without cutting-. They were cleaned by etching and washing, 

 dried and preserved free from impurity as usual, and were used in analyses 



7 Snd 9 " NITRIC ACID. 



For preliminary work, ordinary ''chemically pure'' nitric acid was 

 distilled once, using a platinum condenser and 'Jena glass receiver, the 

 first third of the acid being discarded. For the final analyses this redis- 

 tilled product, already very pure, was once again distilled, as before, the 

 first third being rejected. This distillate gave no test with the nephel- 

 ometer, after dilution. 



\\ J\ J. ILIv. 



As usual in such work, all the water used had been carefully purified by 

 double distillation, once from a fairly strong solution of alkaline potassic 

 permanganate, and then alone. The distillate was caught in Jena glass 

 receiving flasks which were provided with special adapters to prevent 

 the access of dust. The first and last fractions were always rejected, only 

 the middle portion being used. The connection of the Jena glass boiling- 

 flask with the pure tin condenser was made without rubber or cork. 1 

 Such water gave even- evidence of sufficient purity, as has been frequently 

 pointed out. TH LABORATORY . 



A very essential precaution in every stage of this work was to effectually 

 exclude hydrochloric acid. Accordingly, all the preparations of the bro- 

 mide were carried out in a private laboratory, used especially and only 

 for this work, and wholly free from the interference caused by large 

 classes or other researches. Great care was taken not to allow any 

 hydrochloric acid in the room ; indeed, not a drop was used there during 

 the entire research. A separate ventilating fan prevented fumes from 

 other laboratories from interfering with the work. 



UTENSILS. 



As usual, great care was taken to avoid the use of any vessels which 

 under the given conditions might be attacked and thus pollute the sub- 

 stance in hand. "\Yhen glass and porcelain were not harmful, as in the 

 early stages of the preparation of silver, such receptacles were employed ; 

 but in the purification of the substance for analysis, platinum and quartz 

 were always used. 



iRichards, Proc. Am. AcacL 30, 330 (1S94). 



