60 MOLECULAR WEIGHT AND POLYMERISM 



since Henry's law of absorption applies to them (p. 31) ; 

 for iodine the same has been shown by measurements of 

 the freezing point ' . Again, the monatomicity of the metals 

 (p. 35), deduced from their vapour densities, is found to 

 be true for solution in mercury 2 , whilst phosphorus and 

 sulphur appear as tetr- and oct-atomic respectively, as in 

 the form of vapour. 



Kesearches on the molecular weights of the elements can 

 thus be carried much further, since the non- volatility of the 

 metals is here of no consequence, and so nearly all the 

 metals have been studied in solution in tin, and have mostly 

 proved monatomic. 



As to compounds, the tendency of acetic and formic 

 acids to form double molecules, shown by the vapour 

 density, recurs in many (hydroxyl-free) solvents 3 . 



Larger molecules in hydroxylic compounds. What does 

 not appear from the investigation of gases and vapours is 

 the tendency of nearly all hydroxylic compounds to form 

 double molecules when in somewhat concentrated solution, 

 as was mentioned for formic and acetic acids. This is found 

 to be the case in general for organic acids, for the alcohols, 

 and for water. The presence of double molecules, however, 

 depends on the solvent, being found only in hydroxyl-free 

 liquids, such as the hydrocarbons, chloroform, and carbon 

 disulphide. In hydroxylic solvents, phenol, acetic acid, 

 water, the reduplication does not appear, either because 

 of a dissociating action of the solvent, or because the dis- 

 solved body forms molecular complexes with the solvent, 

 in which only one molecule of dissolved occurs. Hence in 

 practice, as the smallest molecular weight answering to the 

 chemical behaviour is sought, it appears that hydroxylic 

 solvents such as acetic acid are to be preferred, especially 

 when dealing with hydroxylic bodies. 



1 Beckmann, Zeitschr. f. Phys. Chem. 5. 76 ; 17. 107 ; 22. 614 ; Helff, 1. c. 

 12. 219; Aronstein and Meihuizen, Verh. Kon. Akad. Amsterdam, 1898. 

 3 Ramsay, Zeitschr. f. Phys. Chem. 3. 359 ; Tammann, 1. c. 3. 441. 

 3 Beckmann, 1. c. 2. 742. 



