THE RATIO OF SODIC TO ARGENTIC CHLORIDE. 35 



It is needless to say that these experiments were carefully carried 

 on in complete darkness, in order to avoid the known disturbing effect 

 of photochemical action. The temperature was usually 20 C. Un- 

 doubtedly this condition is important, since the aggregation of argentic 

 chloride is greatly affected by change of temperature. 



THE RATIO OF SODIC TO ARGENTIC CHLORIDE. 



Two obvious means are available for determining the combining 

 weight of sodic chloride one, by weighing the amount of argentic 

 chloride which may be made from a given weight of this salt ; the 

 other, by discovering the weight of silver needed to precipitate all its 

 chlorine. Both these methods involve a consideration of all the 

 special points mentioned in the preceding sections, and both are essen- 

 tially gravimetric, although both involve the use of the nephelometer 

 in order to estimate the last traces of silver or chlorine in the super- 

 natant liquid. 



The actual investigation of these two methods proceeded simul- 

 taneously in order to economize the time during the long delays to 

 which each was frequently subject. Indeed, in some of the previous 

 work in this laboratory, where the pure materials were scarce, the two 

 methods were actually combined in the same analysis, by first deter- 

 mining the weight of silver needed to precipitate the chloride, and 

 then weighing the chloride produced. This last procedure is never- 

 theless somewhat objectionable on account of its complication. It is 

 moreover subject to slight opposing errors ; the avoidance of one of 

 these involves the admission of the other, as has already been pointed 

 out in a preceding communication.* The magnitude of the uncer- 

 tainty thus caused is not great, and the final error involved is insig- 

 nificant even in atomic weight investigation of ordinary caliber, usually 

 not exceeding o. 01 per cent; but in the present case extraordinary 

 accuracy was sought. Hence, although the methods were studied 

 simultaneously, they were never combined in the same analysis. For 

 this reason the work is easily divided into two sections, each treating 

 one of the two ratios named above. Of these it is convenient to dis- 

 cuss first the ratio of the chlorides to one another ; and this discussion 

 is taken up in the present chapter. 



After the preliminary experiments had indicated the nature of the 

 problems involved, a single specimen of pure sodic chloride was used 

 for analysis until all the details of analysis were perfected. Then the 



*Richards and Archibald on the atomic weight of caesium. Proc. Am. Acad., 

 38, 443 (1903). 



