254 PRINCIPLES OF CHKMISTRY 



pounds part with their ammonia even when left exposed to the air, but 

 others only do so at a red heat ; many give up their ammonia when 

 dissolved, whilst others dissolve without decomposition, and when 

 evaporated separate from their solutions unchanged. All these facts 

 only indicate that ammoriiacal, like aqueous, compounds dissociate with 

 greater or lesser facility. 19 Certain metallic oxides also absorb ammonia 

 and are dissolved in ammonia water. Such are, for instance, the oxides 

 of zinc, nickel, copper, and many others ; the majority of such compounds 

 are unstable. The property of ammonia of combining with the oxides 

 of certain metals explains its action on certain metals. 20 For this 

 reason, copper vessels are not suitable for holding liquids containing 

 ammonia. Iron is not acted on by such liquids. 



The relation of ammonia and water to salts and other substances 

 becomes especially clear in the case when the salt is capable of combining 

 with both ammonia and water. Take, for example, copper sulphate, 

 CuSO 4 . As we saw in Chapter I., it gives with water blue crystals, 

 CuSO 4 ,5H 2 O ; but it also absorbs ammonia in the same molecular 

 proportion, forming a blue substance, CuSO 4 ,5NH 3 , and therefore the 

 ammonia combining with salts may be termed ammonia of crystallisation. 



Such are the reactions of combination proper to ammonia. Let us 

 now turn our attention to the reactions of substitution proper to this 

 substance. If ammonia be passed through a heated tube containing 

 metallic potassium, then hydrogen is evolved, and a compound is 

 obtained containing ammonia in which one atom of hydrogen is re- 

 placed by an atom of potassium, NH 2 K (according to the equation 

 NH 3 + K = NH 2 K + H). This body is termed potassamide. We shall 

 afterwards see that iodine and chlorine are also capable of directly 

 displacing hydrogen from ammonia, and of replacing it ; we shall also 

 see that in hydrocyanic acid, NCH, carbon has replaced hydrogen. 

 Hence the hydrogen of ammonia may be replaced in many ways by 

 different elements. If in so doing NH 2 remains, the resultant sub- 

 stances are called amides, if only NH imides, and those in which the 

 whole of the hydrogen is displaced are termed nitrides. It may be 

 imagined that the albuminous substances that is, the complex organic 



19 Isambert studied the dissociation of ammoniacal compounds, as \\c have seen in 

 Note 8, and he showed that at low temperatures many salts are able to combine with a 

 still greater amount of ammonia, which proves an entire analogy with aqueous coin- 

 pounds ; and as in this case it is easy to isolate the definite compounds, and as the least 

 possible tension of ammonia is greater than that of water, therefore the ammonim al 

 compounds present a great and peculiar interest, both as a means for explaining the 

 nature of aqueous solutions, and as a confirmation of the conception of the formation of 

 definite compounds in them ; for these reasons we shall frequently turn to these com- 

 pounds in the further exposition of this work. 



2 Chapter V. Note 2. 



