THE MOLECULAR THEORY 



351 



temperatures the molecules POLYMERISE, i.e. collect together 

 into larger groups of atoms, the average number of atoms in 

 the molecule at the boiling point being about 6. l 



The smallest weight of carbon in the table is 1 2 ; this is 

 therefore the atomic weight. In the case of carbon, Can- 

 nizzaro points out that " since we cannot determine the 

 vapour-density of free carbon, we have no means of knowing 

 the weight of its molecule, and thus we cannot know how 

 many times the atom is contained in it." But this does not 

 in the least affect our knowledge of the atomic weight of 

 the element, since " the knowledge of the weight of the 

 molecule of this last would merely add a datum more to 

 those which are already sufficient for the solution of the 

 problem" (A.C.R. XVIII. 14). 



7. PHOSPHORUS, ARSENIC AND MERCURY. P = 3i, As 

 Hg= 200 



75, 



(See also under hydrogen and chlorine. ) 



The smallest quantities of these elements which are found 

 in the molecules of their compounds are 31, 75 and 200 

 respectively. The vapours of phosphorus and arsenic are 

 therefore composed of " tetratomic " molecules which are 

 subdivided into quarter-molecules by combination with 

 hydrogen or chlorine. The vapour of mercury on the other 

 hand is composed of " monatomic " molecules, which 



1 Recent work has indicated that the vapour is a mixture of S 8 and 

 S 2 , the existence of S 6 being very doubtful. 



