RICHARDS. — INTERNATIONAL ATOMIC WEIGHTS. 173 



vantages of this international standard. In the first place, it is evident 

 that the question is a practical one, not a theoretical one. If Prout's 

 ancient hypothesis seemed at all probable, there would indeed be a strong 

 reason for assuming hydrogen as unity ; but Prout's hypothesis cannot 

 now claim serious consideration, at any rate in its original form. No 

 other theoretical reason for calling hydrogen exactly 1.000 is known to 

 me. What, then, are the relative practical advantages to be gained by 

 taking hydrogen or oxygen as the standard ? In the first place, the 

 precise quantitative analysis of compounds containing hydrogen is a very 

 difficult matter, and water is the only one which has been adequately 

 studied. Hence nearly all atomic weights must be referred to hydrogen 

 through the medium of oxygen ; and if the ratio H : O is found to be 

 even a little in error, all other values must be recalculated. Morley's 

 work on this ratio is indeed magnificent, and it is not likely that his 

 accuracy can be surpassed for a long time ; nevertheless the principle 

 still remains. Oxygen, on the other hand, has been directly compared 

 with many metals, as well as with potassic chloride and similar salts 

 obtainable from the chlorates and their analogues. Hence from the 

 point of view of directness of comparison, oxygen is to be preferred. 

 Silver might be even better, as Erdmann and Volhard point out in their 

 replies to the circular letter ; but the question does not concern the start- 

 ing of an entirely new system, but rather the choice between two old 

 ones. 



Another point to be considered is the effect of the decision upon the 

 data contained in the past literature of chemistry. Any change which 

 might confuse the understanding of the work of the past would be 

 indeed a grievous one; and a change to O = 15.879 could not but have 

 this effect. Little or nothing has been written with the assumption of 

 this standard, while a great bulk has been written with the assumption 

 O = 16. The confusion caused by the inaccurate value O = 15.96 is 

 quite bad enough, without the introduction of a new stumbling-block. 

 Moreover, in the gas constant and a multitude of other physico-chemical 

 constants the value — 16 enters, and a change in this standard would 

 complicate the use of a great mass of valuable literature of this kind. 



Another, although somewhat trivial, reason why oxygen should be 

 taken as 16 is because in that case a somewhat larger proportion of the 

 atomic weights approximate whole numbers than would be the case 

 otherwise. 



The chief objection to the proposed standard is a pedagogical one. It 

 is claimed that confusion is caused in the mind of the elementary student 



