12 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



PREPARATION OF PURE MATERIALS. 



WATER. 



The traces of organic impurity in the distilled water of the labor- 

 atory were eliminated by redistilling from a weakly alkaline solu- 

 tion of potassic permanganate, using at first a glass condenser and 

 rejecting the first quarter of the distillate. Water thus prepared was 

 pure enough for preliminary nephelometric work, when the presence 

 of a trace of alkali could do no harm ; it was usually kept in Jena 

 flasks. In cases where the greatest purity was desired it was distilled, 

 of course, once further, and for the nephelometric experiments it was 

 kept in a closed bottle provided with a siphon and protected from pos- 

 sible traces of hydrochloric acid by a tube containing sodic hydroxide. 

 For final preparations and accurate analyses, where silica and alkali 

 must be excluded, the water was condensed and collected during the 

 third distillation wholly in platinum. Care was taken to exclude dust 

 in all stages of the proceedings; for, although visible dust might 

 weigh little, its presence in silver halogen salt would produce decom- 

 position and loss of weight in the final fusion. Only one who has 

 attempted to exclude all dust from solutions can appreciate the diffi- 

 culty of doing so. Thorpe and Rodger,* working upon viscosities, 

 found that sealed-in condensers were very helpful in excluding dust 

 from their distillates. We did not go as far as this, but the danger was 

 borne in mind at every stage of the work and guarded against. 



HYDROCHLORIC ACID. 



For the final work the purest acid of commerce was treated with 

 a few crystals of potassic permanganate, diluted, and boiled. This 

 should have expelled bromine and iodine, and must have oxidized 

 any trace of organic matter which might have been present. The 

 oxidation and boiling were repeated ; the solution was fractionally 

 distilled; and for the succeeding treatment the middle fraction was 

 selected and subjected to another distillation, in which a platinum con- 

 denser and receiver were employed. As a matter of fact, most of these 

 precautions were found to be unnecessary, since salt made, as described 

 on page 16, from the best "chemically" pure acid of commerce only 

 once redistilled in platinum, without any further precautions, gave the 

 same results as the purest material. 



Phil. Trans., 185 A, 432 (1894). 



