348 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



It is seen that the quantity of hydrogen in the molecule is 

 always a half-molecule or some integral number of half- 

 molecules ; the half-molecule of hydrogen appears therefore 

 to represent the extreme limit to which the subdivision of 

 the molecule can be carried. Cannizzaro concludes that 

 "The atom of hydrogen is contained twice in the molecule 

 of free hydrogen" (A.C.R. XVIII. n). 



2. OXYGEN COMPOUNDS. O=i6. 



(See also under Carbon. ) 



The smallest weight of oxygen in the molecule is 16. 

 This is the weight of the oxygen atom and is now taken as 

 the standard of comparison for all other atomic weights. 

 The molecule of oxygen gas has weight 32 ; it is therefore 

 a " diatomic " molecule containing two atoms, as represented 

 by the formula O 2 ; but ozone, prepared from it by electri- 

 fication, is seen to be a " triatomic " modification, containing 

 three atoms in the molecule, as represented by the formula 

 (X 



