96 CHEMICAL PROPERTIES AND COMPOSITION 



off sulphur. Moreover the density of sulphur vapour 

 (Part II, pp. 36, 60) indicates a composition $ 8 , which how- 

 ever changes at higher temperature into $. 2 . 



In oxygen the tendency to self-combination is further 

 reduced, and even the compounds that contain two con- 

 nected oxygen atoms are not very stable, whilst free oxygen 

 when triatomiCj in the form of ozone, is well known to be 

 explosive. 



It may be remarked finally that the tendency to self- 

 combination is so far connected with the positive or nega- 

 tive character, that it appears only in the intermediate 

 elements. The table, p. 93, shows this clearly ; left and 

 right are sodium and chlorine, strong positive and nega- 

 tive, of which the former occurs in monatomic molecules, 

 whilst the latter decomposes into such on heating : then 

 come hydrogen and oxygen, which are quite stable in the 

 diatomic form, and of which oxygen is known as triatomic ; 

 then follow nitrogen and phosphorus, of which the former 

 is known e. g. as azoic acid N 3 H with three atoms united 

 together, whilst phosphorus occurs as tetratomic molecules. 

 In the middle are sulphur and carbon. 



As is usually the case, this effect of affinity is strongest 

 for small atomic weight, as is seen in the specially strong 

 tendency of carbon to combine with itself, and in the rela- 

 tive ease with which the diatomic molecules of iodine 

 break up as compared with the stability of chlorine in this 

 respect. 



4. Velocity of Reactions *. 



The elements show characteristic differences in the very 

 diverse facility with which reactions occurring in their 

 compounds take place. Carbon in particular is distin- 

 guished by its striking ' inertia ' in this respect. 



Even in the simplest cases, on going into combination 

 with other elements this inertia of carbon is apparent, in 

 the first place, in its high temperature of inflammation. 



1 Ansichten tiber organische Chemie, ii. 224. 244. 



