106 Prof. H. E. Armstrong on the Determination of 



Crafts, is resolved into monatomic molecules with compara- 

 tive ease, although far less readily than nitrogen peroxide is 

 resolved into nitrogen dioxide ; but the entire behaviour of 

 oxygen, nitrogen and carbon would seem to show that the 

 stability of their molecules is enormously greater than that of 

 diatomic iodine molecules. Thomsen's conclusion also appears 

 the less likely to command acceptance when the mode of de- 

 ducing the values for nitrogen and oxygen is considered, and 

 it is remembered that the value for nitrogen is nothing more 

 than the heat of dissociation of nitrogen peroxide. All the 

 facts, so far as I can interpret them, appear to me to show 

 that nitrogen peroxide and nitrogen are to be placed at the 

 very opposite extremes in the scale of molecular stability. 

 Another instance of the unsatisfactory nature of the argument 

 upon which Thomsen's determinations of the affinity values 

 for nitrogen compounds is based is afforded by nitric oxide, 

 the formation of which, from its atoms, would, it is supposed, 

 take place without evolution of heat. Meyer and Ziiblin have 

 shown, however, that this gas is stable at 1200°, although it 

 is entirely decomposed at about 1700°. 



(28) It remains to consider those results which are inde- 

 pendent of any correction in the value of/. C ly &c. Among 

 the most interesting are those relating to the chlorides (§9), 

 and to isomeric alcohols (§ 10). As regards the former it 

 will be recollected that whereas the addition of a single atom 

 of chlorine is attended by a heat-evolution of only 13500 

 units, that of tivo involves the production of no less than 

 2.16500 units, that of three of 13500 + 2.16500 units, and that 

 of four of 2.13500 + 2.16500 units. It appears to me that 

 the greater amount of heat developed in the formation of the 

 symmetrical ^-derivative (symmetrical as regards the relation 

 of the chlorine atoms to the carbon atom or atoms) is pro- 

 bably due to the partial neutralization of the (residual) affinity 

 of the one chlorine atom by that of the other. It is unfortu- 

 nate that no chlorine compound, in which the two chlorine 

 atoms are associated with non-contiguous carbon atoms, has 

 been examined. According to the view here put forward it is 

 scarcely to be expected that in such a case there will be the 

 extra development of heat which Thomsen has noticed, 

 although it is conceivable, however, that the association of an 

 even number of chlorine atoms either with a single carbon 

 atom, or with contiguous or symmetrically placed carbon 

 atoms, may involve an extra outgoing of affinity. 



(29) Passing to the alcohols, we have to note Thomsen's 

 conclusion that the primary have the highest, and the isomeric 

 tertiary the lowest heats of combustion, the secondary occu- 



