RICHARDS. — THE STANDARD OF ATOMIC WEIGHTS. 179 



to silver lies in the fact that it cannot be directly used in the demonstra- 

 tion of Avogadro's rule. Moreover, one is in doubt as to the value to 

 assign to this element, supposing that it should be selected as the stand- 

 ard. According to Erdmann's earlier arguments, logically followed out, 

 one should make silver 100.000, but this would cause hydrogen to be less 

 than unity. If silver is taken as 107.11, hydrogen would be 1.000 at 

 the present time, but what it might be in the future no one can predict, 

 since hydrogen is compared with silver at present only in a roundabout 

 fashion. Hence each of these assumptions would bring with it a further 

 disadvantage besides that attending the immediate inconvenience of using 

 new values. 



The most important argument used by the minority is the pedagogic 

 one. It is contended that the uneven value for hydrogen, 1.0075, com- 

 plicates the explanation of the very important rule of Avogadro. If 

 this were true, it would indeed be worthy of consideration, but according 

 to my experience there is no difficulty in the matter. 



For some time I have abandoned the comparison of specific gravities 



as a means of demonstrating Avogadro's rule. I have used instead the 



densities of gases and vapors, — that is, the actual weights of a litre of 



the several substances at 0°C, or at 273° C or at 546°C. This seems 



to be a more successful method, probably because density has concrete 



dimensions, and is not a numerical abstraction as specific gravity is. 



1.97 x 

 The student at once comprehends the equation of ratios -^— - = • 



If the exact experimental values for the densities of the two gases arc* 

 given, the solution of this equation gives the student not only the ob- 

 served molecular weight of carbon dioxide, but also an insight into the 

 extent of the actual deviations from Avogadro's rule. Since the intro- 

 duction of this method of presentation, I have had far less trouble, and 

 far more successful examination results, than were formerly obtained. 

 The student usually learned by heart the old rule, "The molecular 

 weight equals twice the specific gravity," without understanding it. 

 Because the density-method would serve equally well with any gas 

 used as a standard, the pedagogic argument against II = 1.0075 seems 

 to me illusory. 



The argument just discussed has led the Committee of the German 

 Chemical Society to an action which seems to me exceedingly unfortu- 

 nate, — namely, the publication of two tables of atomic weights. This 

 action has already been criticised by Krister and others. Either table 

 alone, supported by suitable weight of opinion, would have been vastly 



