Mr. M. M. Pattison Muir on Chemical Classification, 97 



Very striking examples of the class of compounds under 

 consideration are met with in the combinations of certain of 

 the nitrogen group of elements. Sal ammoniac is one of the 

 most common of these compounds. If this body be really a 

 chemical compound of the composition expressed by the for- 

 mula NH4 CI, then nitrogen is a pentad element. But if this 

 be so, why does NH^ CI split up, on heating, into NH3 + HCI ? 

 Because, answers one school of chemists, although nitrogen is 

 a pentad, yet its " affinities " or '^ combining-powers " are of 

 unequal values ; three are stronger than the other two. Or 

 the answer may be stated in a slightly altered form : The 

 nitrogen atom is capable of exerting force in five directions ; 

 but the amount of force exerted is not the same for each direc- 

 tion. Another school gives an answer somewhat different 

 from this. They say that the combining-powers of nitrogen 

 are all, originally, of equal value, but that, after three of 

 these have been saturated, the introduction of the new atoms 

 has so modified the conditions as to cause the remaining com- 

 bining-powers to be weaker than they originally were. These 

 two answers, although differing somewhat, are yet at one in 

 regarding sal ammoniac as a compound the same in kind as 

 water, or hydrochloric acid, or any other body which is known 

 to exist undecomposed in the state of gas. But a third school 

 of chemists answers the question,Why does sal ammoniac split 

 up when heated ? in a totally different manner : — It splits up so 

 readily because it is of an essentially different structure from 

 those compounds which, when caused to become gases, remain 

 undecomposed: nitrogen is really a triad ; ammonia (NH3) is 

 a saturated molecule ; CI2 is also a saturated molecule ; but these 

 two molecules are capable of exerting upon each other a certain 

 degree of attraction whereby they are held together, presenting 

 the semblance of one compound molecule, NH4 CI, which is, 

 however, really two molecules. Now the first view, viz. that 

 which regards nitrogen as a pentad element and sal ammoniac 

 as a true atomic compound, certainly allows us to explain the 

 general analogies of the ammonium salts more readily than the 

 other theory, which regards the nitrogen atom as trivalent. If 

 the latter hypothesis be correct, then ammonium sulphate must 

 be a molecular compound of NH3 and Hg SO4 ( = (NH3)2 H2 SO4). 

 But the analogies between this salt and potassium or sodium 

 sulphate are many and well marked. We can scarcely, how- 

 ever, regard potassium sulphate as other than a salt of sulphu- 

 ric acid — that is, a body in which the hydrogen of the acid is 

 really replaced by the metallic atom potassium ( = K2 SO4). 



But although the first view of the composition of sal ammo- 

 niac has this and manv other points in its favour, which I can- 



Phil Mag, S. 5. Vol, 4. No. 23. Aug, 1877. H 



