186 THE ATOMIC WEIGHTS. 



in such a way iliat only SO, could escape. 2.203 grammes 

 of AsoOgjthus treated, gave a loss of 1.0G9 of SO,. Hence 

 As = 74.840. This is a close estimation ; but, being drawn 

 from a single experiment, has so little weight that it need 

 not be included in our final general mean. 



In 1845 Pelouze* applied his method of titration with 

 known quantities of pure silver to the analysis of the tri- 

 chloride of arsenic, AsClg. Using the old Berzelian atomic 

 weights, and putting Ag = 1349.01, and CI = 443.2, he 

 found in three experiments for As the values 937.9, 937.1, 

 and 937.4. Hence 100 parts of silver balance the following 

 quantities of AsClg : 



56.029 

 56.009 

 56.016 



Mean, 56.01S, d= .004 



Later, the same method was employed by Dumas.f whose 

 weighings, reduced to the foregoing standard, give the fol- 

 lowing results : 



4.298 grm. AsClj = 7.673 grm. Ag. Ratio, 56.015 



5.535 " 9.880 " 56.022 



7.660 " 13.686 " 55-970 



4.680 " 8.358 " 55.993 



Mean, 56.000, rh .008 



The two series of Pelouze and Dumas, combined, give a 

 general mean of 56.014, ± .0035, as the amount of AsClg 

 equivalent to 100 parts of silver. Hence As = 74.829, ± 

 .048, a value closely agreeing with that deduced from the 

 single experiment of Berzelius. 



The same process of titration with silver was applied by 

 Wallacel to the analysis of arsenic tribromide, AsBrg . This 

 compound was repeatedly distilled to ensure purity, and 

 was well crystallized. His weighings show that the quanti- 



* Compt. Rend., 20, 1047. 



t Ann. Chim. Phys., (3,) 55, 174. 1859. 



X Philosophical Magazine, (4,) i8, 279. 



