100 THE ATOMIC WEIGHTS. 



Reducing these ratios with the subjoined data : 



O = 15.879, ± .0003 Na = 22.881, ± .0046 



Ag =: 107. 108, rir .0031 15a ^ 136.392, =h .0086 



.S = 31.828, zb .0015 AgBr = 186.452, dr .0054 

 C = 11.920, =b .0004 



We have nine values for the atomic weight of copjjer. Since ratio (7) 

 depends upon one experiment only, it is necessary to assign the value 

 derived from it arbitrary weight. This will be taken as indicated b}' a 

 probable error double that of the next highest, obtained from ratio (2). 

 The values then are as follows : 



From (i) Cu ^ 62.869, zb .0034 



From (2) " =63.022,^.0070 



From (3) " = 63.070, ± .0030 



From (4) " ^ 63.003, zh .0042 



From (5) " =63.127, zh .0051 



From (6) " ^ 63. 128, zh .0050 



From (7) " = 63.215, zfc .0140 



From (8) " ^ 63.110, zh .0032 



From (9) " = 63. 1 14, zb .0020 



General mean Cu = 63.070, zb .0012 



If = 16, Cu = 63.550. If we include Hampe's analyses of copper 

 sulphate, which gave Cu = 62.889, ± .0085, the general mean becomes 

 Cu = 68.046, ±.0011. 



The foregoing means, however, are significant only as showing the 

 effect and weight of the older data upon the newer determinations of 

 Richards. The seventh of the individual values is also interesting, for 

 the reason that the experiment upon whicli it depends was published by 

 Richards jirevious to his investigation of the atomic weight of barium. 

 With the old value for Ba, 137, it gives a value for copper in close agree- 

 ment with Richards' other determinations. With the new value for 

 barium it becomes discordant, although its weight is so low that it i)ro- 

 duces no appreciable effect upon the final mean. 



Rejecting values 1 to 4, inclusive, the remaining five values give a gen- 

 eral mean of 



Cu = 63. 119, zb .0015. 



If = 16, this becomes 63 600, and in the light of all the evidence 

 these figures are to be preferred. If, again, we combine with this mean 

 the results of Richards' work on the oxide and sulphate of copper, the 

 final value becomes 



. Cu = 63.108, zb .0013, 



and with = 16, 63.589. This departs but little from the previous mean 

 value, but it includes data which render it, in all probability, a trifle too 

 low. The value Cu = 63.119 will be regarded as the best. 



