492 SCIENCE PROGRESS 



of the sphere, the model is indicative of the tetradic condition 

 of oxygen, the intensity of the secondary or extra valency 

 being indicated by the extent to which the rods project. For 

 example : 



^r 



This method of representation is not applicable, however, 

 to chlorine and nitrogen, as these elements both increase in 

 valency to the same extent as oxygen does, that is to say, to 

 the extent of two units, two latent affinities becoming active in 

 both cases when the normal or hydride valency is exceeded. 

 At present we have no explanation to offer of these peculiarities. 

 The formula of the several hydrides considered are best written, 

 therefore, simply as follows : 



HC1< H 2 0< H 3 N< H 4 C 



to indicate the existence of unsaturated affinities and their 

 number in the dominant element in the case of three of the 

 four compounds. 



It will be desirable now to consider the nature of the 

 compounds formed by the association of the hydrides of 

 chlorine, oxygen and nitrogen. That formed from hydrogen 

 chloride and ammonia may be taken first, as it is a well-defined 

 stable compound derived from single molecules of the two 

 substances : 



NH 3 + HC1 = NH 4 C1 



The composition of this compound, ammonium chloride, has 

 been the subject of much discussion in days gone by. In the 

 first place, it is noteworthy that it resembles most closely in 

 all essential particulars the haloids or metallic halides such as 

 common salt, NaCl, potassium chloride and similar substances — 

 the compound radicle ammonium, NH 4 , playing a part in it 

 exactly similar to that of the simple metallic radicle in these 

 latter. From this it may be argued that when interaction takes 

 place the hydrogen and chlorine in hydrogen chloride part 

 company and combine separately with the nitrogen of ammonia. 

 If this were not the case and the hydrogen atom of the chloride 

 retained its individuality in the new compound, it should be 



