16 MOLECULAR WEIGHT AND POLYMERISM 



a silver salt C 2 H 3 Ag0 2 containing at least two carbon 

 atoms in the molecule, n = i for acetic acid is practically 

 excluded, and n=z rendered very probable. 



These chemical methods, though mostly sufficient in 

 practice, are exposed to the logical objection that they 

 depend on products of reaction, and that during the reaction 

 a total transformation of the molecular complex might 

 occur ; e. g. the formation of acetic acid might be 



C 2 H 6 + 20 = 2CH 2 + H 2 0, 



and on the other hand can at most only give a probable 

 minimum size for the molecule; it is therefore fortunate 

 that the physical methods are suitable for directly testing 

 the molecular weight of the bodies in question. 



In the following collection of physical methods for 

 determination of molecular weight, a selection has been 

 made in accordance with the plan of the work. On the 

 one hand are the methods based on Avogadro's law, and 

 applicable to gases and solutions ; on the other are methods 

 applicable to liquids, of which that of Eotvos, Ramsay, and 

 Shields, based on the change of surface tension with tem- 

 perature, is an example ; on account of the empirical 

 character of the latter they will be found in Part III, 

 which is concerned with the relations between properties 

 (e. g. capillary constant) and composition (and therefore 

 molecular weight). We shall therefore confine ourselves 

 here to : 



i. Molecular weight determination in dilute gases. 



2. solutions. 



3. solid solutions. 



i. MOLECULAK WEIGHT DETERMINATION IN DILUTE 



GASES. 



A. Avogadro's Law. 



It is well known that the law on which the determination 

 of molecular weights in gases is based, as stated by Avogadro, 

 is to the effect that equal volumes of dilute gases, measured 



