94 CHEMICAL PROPERTIES AND COMPOSITION 



Fig. 4, in which the heat values for sodium and chlorine, 

 on combination with sulphur, oxygen, chlorine, iodine, 

 hydrogen, and sodium, are represented by the lines I and n. 



It should be added that the contrast which we have 

 seen to exist between sodium and chlorine in their facility 

 for associating with electricity, i. e. for forming ions, 

 reappears in these thermal values, and that the evolu- 

 tion of heat in the combination of two elements goes 

 hand-in-hand with their facility for associating with oppo- 

 site electrical charges. One would be inclined from this 

 to think that in the more marked phenomena of affinity 

 electricity plays the leading role, and that whilst a pair of 

 similar atoms such as the chlorine in the chlorine mole- 

 cules are held together essentially by their direct mutual 

 action, in sodium chloride, on the other hand, the sodium 

 and chlorine hold fast to a charge of negative and positive 

 electricity respectively, which then, by their mutual attrac- 

 tion, unite the whole. 



Again, the two opposite indications of affinity depending 

 on electric charges are best seen in univalent elements, 

 and especially those with small atomic weights, and large 

 atomic volumes, i. e. when the action of mass is smallest. 



