5o 



ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



found to be more accurate. A ratio of ioo millimeters to 80 millimeters 

 was found to signify an excess or deficiency of o. 15 milligram of silver 

 per liter, and other amounts in proportion. Since the average of many 

 readings was probably accurate within 2 per cent and the volume rarely 

 exceeded a liter, it would appear that the end-point was determined 

 within 0.02 milligram of silver. This does not, of course, prove that 

 the results are accurate to within this small margin, because other 

 experimental details may have led to larger error. The difference 

 between the maximum and minimum results corresponds, indeed, to 

 a much greater uncertainty, as will be seen. Hence the end-point is 

 determined precisely enough for our purpose. 



The final series of experiments, given below, includes only those 

 experiments in which every known source of error was avoided. In 

 order to illustrate the details of the method, the full record of a single 

 experiment, in so far as it does not repeat what has already been described, 

 is here given. 



Experiment 53. 



Grains. 



Weight of sodk chloride (in vacuum) (Sample I) 5.08685 



Weight of silver (in vacuum) (Sample O) 9.38819 



These were dissolved and mixed on February 2, 1904. After two 



hours' agitation the mixture stood until next day, when an examination 



was made with the nephelometer. Two test tubes were filled with the 



mother liquor by means of a clean pipette, withdrawing 0.07 liter for 



this purpose. The two solutions were very nearly equal in opalescence 



With occasional shaking the mixture was therefore allowed to stand 



yet another day, in order to insure the thorough solution of possibly 



adsorbed material. The subsequent proceedings are summarized 



below : 



February 4 (0.07 liter withdrawn). 



This reading indicates a slight deficiency of silver. Hence 0.15 

 milligram of silver in solution was added to the residual mother liquor, 

 and the mixture was occasionall3 r shaken. 



