4 8 



ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



nitrate, having first one and then the other in excess, the speed of 

 formation was not like that observed when a solution of argentic 

 chloride was treated with the two electrolytes. In the former case, the 

 maximum opalescence was attained more slowly and the tube con- 

 taining an excess of chloride always became cloudy much more rapidly 

 than that containing an excess of silver, while in the latter case the 

 difference was much less marked. This adds to the evidence that the 

 dissolved argentic chloride is appreciably colloidal, and that the forma- 

 tion of this colloidal portion requires time. 



The possible effect of light upon this end-point was another cir- 

 cumstance of interest, although not of immediate bearing on our 

 quantitative work, since actinic light had been scrupulously excluded. 

 In order to ascertain the effect of light the solution already tested in 

 observations 65, 66, and 67 was agitated in the flask for an hour at a 

 distance of about 30 cm. from a white incandescent electric light. Ob- 

 servations 68 and 69 were made with the mother liquor. It was then 

 agitated in diffused daylight for another hour, an exposure which made 

 the precipitate decidedly bluish, and observations 70 and 71 taken. 



The solution thus gave the same results in the nephelometer as 

 before. Bottger, in his study of the solubility by the conductivity 

 method, also observed that the conductivity did not sensibly alter until 

 the conductivity vessel was exposed to strong light for a considerable 

 length of time. A slight coloring of the precipitate made no dif- 

 ference.* The explanation must be that chlorine thus set free does not 

 ionize. According to Vogel.f silver subcloride is insoluble in very 

 dilute nitric acid. Hence neither an excess of ionized silver nor 

 chloride should appear in the mother liquor. L,ight, therefore, could 

 not have been the cause of any of the variations in preceding observa- 

 tions, and its complete exclusion had been supererogatory. 



*Zeit. phys. Chem., 46, 521 (1904). 



t Gilbert's Annalen, 72, 286. 



