80 SMITH'S INTERMEDIATE CHEMISTRY 



a smallest unit weight of exactly 1. Now hydrogen is the element 

 which enters in the smallest proportions into chemical combination, 

 and we have seen that in 22.4 1. of some compounds there is as 

 little as 1.008 g. of this element (see table, p. 76). 



If a slightly smaller unit volume (22.2 1.) had been chosen, 

 hydrogen would possess a unit weight of exactly 1. Why was 

 not, it will be asked, this volume selected instead of 22.4 1. as our 

 standard? It was, in fact, at one time the standard, the unit 

 weight of oxygen being then about 15.9 and the unit weights for 

 all of the other elements being correspondingly reduced. The 

 transfer to the present scale = 16 was made for a purely prac- 

 tical reason; because of the greater activity of oxygen (see p. 27), 

 the exact determinations of the combining proportions of most 

 other elements are generally given by experiment on an oxygen, 

 not on a hydrogen, basis. It so happens, also, that the majority 

 of atomic weights on the scale O = 16 are very close to whole 

 numbers (see table), an aid to calculations being thereby afforded 

 which does not exist under the standard H = 1. Recent work 

 on the " complexity " of elements, it may be added, has given fur- 

 ther justification of our choice of the oxygen standard here adopted 

 (see p. 552). 



The accepted scale is, therefore, that of 32 for the unit weight of 

 oxygen gas; 22.4 1. (the volume of 32 g. of free oxygen) for the 

 chemical unit of volume; and 16 for the unit weight of oxygen in 

 compounds containing that element. The unit weights of all the 

 other compounds and elements are based upon this scale. 



Further Discussion. At this point we have completed the 

 explanation of the derivation of the units of weight, and of the 

 symbols and formulae used in chemistry. We are now in a posi- 

 tion to proceed with the application of these conceptions, as they 

 are developed in Chapter IX. This would be possible, at least 

 so far as strict logic is concerned. But, in shaping our course 

 so as to reach the results by the shortest route, we have omitted 



