THE METHOD OF GAY-LUSSAC. 45 



been reached. It is not strange, therefore, that many eminent experi- 

 menters adopted this method in order to determine many atomic 

 weights. The solubility of silver chloride, however, was a fact which 

 Gay-Lussac, Pelouze, Dumas, and many others, as well as Stas in his 

 earlier experiments, did not consider; in consequence, all used the 

 wrong end -point. In his last experiments Stas, recognizing the 

 solubility of argentic chloride, attained the end-point in two ways. 

 The first method consisted in titrating to the extreme limit with 

 sodic chloride and then running back to the opposite extreme limit 

 with silver nitrate ; the true end-point was assumed to be half-way 

 between the two limits.* The second method consisted in bringing 

 the mother liquor to such a point that two portions of it gave equal 

 opalescent precipitates with silver nitrate and sodic chloride respec- 

 tively ; this was the end-point established by Mulder. Before accepting 

 this latter method, Stas attempted to prove for himself that a solution 

 of silver chloride in water gave an equally intense opalescent effect 

 with an excess of argentic nitrate and sodic chloride. He concluded 

 that the opalescence in the two cases was identical, but at the same 

 time admitted that the opalescent precipitates were extremely unstable 

 and varied continuously in appearance from the moment of precipitation 

 to the time they had settled out completely .f At first, not hoping to 

 improve essentially in this respect upon the methods of Stas, we used 

 this end-point, and after a few preliminary trials made eight careful 

 determinations, using in all 33.1035 grams of pure salt and 61.0898 

 grams of pure silver. The atomic weight of sodium calculated for the 

 average of these experiments was 23.032, with maximum and minimum 

 of 23.040 and 23.023, respectively. 



These experiments were of value as giving practice in the manip- 

 ulation, but the extreme variation was so much greater than it ought 

 to have been that clearly some undiscovered cause of variable error 

 was present in the results. 



A study of all the possible causes of irregularity eliminated all 

 probabilities except the uncertainty of the end-point. The purity of 

 the salt and the silver left nothing to be desired, for the same samples 

 gave constant results when the argentic chloride was weighed. Occlu- 

 sion had been precluded by the use of solutions of not much over 

 deci-normal strength, and the mechanical operations were so extremely 

 simple as to leave little opportunity for error. Hence much time was 



^Oeuv.,1 755. This is unsatisfactory for many reasons. See Richards, Proc. 

 Am. Acad. ,29, 82 (1893). 

 tOeuv.. 1, 158. 



