178 PROCEEDINGS OP THE AMERICAN ACADEMY. 



measuring or weighing the hydrogen which they displace or to which 

 they correspond, but the results of different experimenters are far from 

 concordant. All other elements beside these eight have been referred 

 to hydrogen only with the assistance of oxygen. 



On the other hand, oxygen has been used as the direct standard of 

 reference in countless cases. The determination of oxygen in the chlo- 

 rates, bromates, and iodates may be considered as the starting-point for 

 the calculation of Ag, K, Na, CI, Br, and I, and through them of very 

 many others. Into this remarkable series of experiments, executed in 

 great measure by Stas, the value of hydrogen enters only in the case 

 of ammonic salts. If the atomic weight of nitrogen were certain, we 

 should indeed have here a direct basis of comparison, but unfortunately 

 the value for this element may be as much as 0.05 per cent, or even 

 more, in error. The direct practical determination of the exact com- 

 position of ammonia gas, either by analysis or synthesis, has not yet 

 been accomplished. The value for nitrogen depends largely upon the 

 analysis or synthesis of nitrates, thus making oxygen the essential stand- 

 ard of reference in this case also. The other elements which have 

 been determined more or less accurately by reference to oxygen are as 

 follows : H, C, Cu, Ca (through the carbonate), Pb (through the 

 nitrate), Zu, Cd, Hg, Tl (through the nitrate), Sn, P, As, Sb, Bi, Mo, 

 U, W, Se, Te, Mn, Fe, Ni, Co. If one adds to these all those which 

 are connected less directly with oxygen through the halogen and silver 

 values and the sulphates, all the chemical elements are included in the 

 list. Thus an overwhelming majority of elements is referred more 

 directly to oxygen than to hydrogen. 



Erdmann points out in his recent paper that there are possible causes 

 of error in some of the methods used by Stas for the analysis of chlo- 

 rates. Unfortunately he does not touch upon the very important ques- 

 tion of the percentage effect of these causes of error. It is undoubtedly 

 true that in these cases, as well as in every other case, absolute accuracy 

 was not attained. No analytical method is wholly free from the possi- 

 bility of error, and hence it is vain to expect that any table of atomic 

 weights should be perfectly trustworthy. When the accuracy of Stas 

 has been exceeded in actual fact, it will be time to forsake his results 

 for the newer values. 



Erdmann suggests that silver be chosen as the standard of reference, 

 and the susgestion is one which has some advantasies. On the other 

 hand the tendency which this metal has to absorb oxygen has cast a sus- 

 picion over some of the work in which it was used. A further objection 



