204 NITROGEN BODIES OF MODERN CHEMISTRY. 



we liaA^e still to mention the qnadrivalent atom of carbon. The carbon 

 atom requires four times the space of the hydrogen atom, and tills that 

 space with the weight 12. It bears the symbol ""G. 



The elements, then, with which we^have to deal and effect our ex- 

 changes are represented as follows: 



Fig. 1. 

 1. Hydrogen, ^ 'H. 



2. Oxygen, C'"^ "^• 



3. Nitrogen, COO '"^• 



4. Carbon, COOO ""Q. 



The equivalence of the elements, i. c, the proportion of their volumes 

 compared with hydrogen as a standard, is not an invariable quantity. 

 Expansion and contraction may take place in the atoms ; within certain 

 limits these can increase or decrease in volume. But it is important to 

 observe that this expansion and contraction will always be bipolar; 

 that is, if a univalent atom exi)and it will do this in both directions and 

 become, not bivalent, but necessarily trivalent. The trivalent nitrogen 

 atom may become quinquivalent, but not quadrivalent. Hence, we have 

 the simple rule that the character of the equivalence cannot change 5 if 

 it is expressed by an even number it must still, though the atom con- 

 tract or expand, be expressed by an even number ; and reversely, if the 

 equivalence is expressed by an odd number it must in all cases be so 

 expressed. This will readily be understood if we suppose a unit volume 

 to be added to each side of the symbol oxygen, carbon, and nitrogen-, as 

 given in Fig. 1. In this case the oxygen will become four, the nitrogen 

 five, &c. The trivalent nitrogen atom in some circumstances may 

 become quinquivalent. This expansion of atoms takes place when they 

 are subjected to peculiar chemical actions. 



I now call your attention to a salt which is produced from acetic acid 

 and ammonia, and which is introduced into the pharmacopoeia as the so- 

 called spirit of Mindererus, the acetate of ammonia, or acetate of oxide 

 of ammonium. 



In the delineation of the formulae of chemical compounds, without 

 which a clear understanding of the processes is impossible, this acetate 

 of ammonia is represented in Fig. 4: 



