tfl^ERS 1 



I. MOLECULAR WEIGHT AND 

 POLYMERISM 



As mentioned above, the molecular hypothesis rests upon 

 a basis both chemical and physical. Accordingly the 

 methods for the determination of molecular weight belong 

 partly to chemistry, partly to physics. 



Molecular weight determinations by chemical methods 

 start from the atomistic hypothesis, according to which the 

 molecule is built up of atoms ; e. g. if a compound contain 

 p, q, and r per cent, of carbon (C = i2), hydrogen (H=i), 

 and oxygen (O=i6) respectively, the ratio of the number 

 of atoms a, 6, and c is given by 



-, p r 



a : o : c = : q : p 9 

 12 16 



in which a, b, and c must be whole numbers. This does 

 not, however, allow of fixing a, b, and c, for if the least 

 integral relation has been calculated all the formulae 



(C H, cg n , 



in which n is unity, or some other integer, would satisfy 

 the conditions. 



To determine n therefore requires a second criterion, 

 which in the region of chemistry mostly leads with suf- 

 ficient probability to a decision, but so far not with absolute 

 certainty. It is supplied by the chemical behaviour com- 

 bination and reaction which renders certain molecular 

 weights probable. Thus taking acetic acid 



(CH 2 0) B) 



and remembering that it arises by oxidation of alcohol, for 

 which the simplest possible formula is C 2 H 6 0, and forms 



