8 THE DATA OF BIOLOGY. 



ponents below a bright red heat. Much less stable, however, 

 are several of the oxides of nitrogen. Nitrous oxide, it is 

 true, does not yield up its elements below a red heat; but 

 nitrogen tetroxide cannot exist if water be added to it; 

 t nitrous acid is decomposed by water; and nitric acid not 

 only readily parts with its oxygen to many metals, but when 

 anhydrous, spontaneously decomposes. Here it will 



be well to note, as having a bearing on what is to follow, 

 how characteristic of most nitrogenous compounds is this 

 special instability. In all the familiar cases of sudden and 

 violent decomposition, the change is due to the presence of 

 nitrogen. The explosion of gunpowder results from the 

 readiness with which the nitrogen contained in the nitrate of 

 potash, yields up the oxygen combined with it. The ex- 

 plosion of gun-cotton, which also contains nitrogen, is a sub- 

 stantially parallel phenomenon. The various fulminating 

 salts are all formed by the union with metals of a certain 

 nitrogenous acid called fulminic acid; which is so unstable 

 that it cannot be obtained in a separate state. Explosive- 

 ness is a property of nitro-mannite, and also of nitro-glycerin. 

 Iodide of nitrogen detonates on the slightest touch, and often 

 without any assignable cause. And the bodies which explode 

 with the most tremendous violence of any known, are the 

 chloride of nitrogen (ISTClg) and hydrazoic acid (N^gH). 

 Thus these easy and rapid decompositions, due to the chemical 

 indifference of nitrogen, are characteristic. When we come 

 hereafter to observe the part which nitrogen plays in organic 

 actions, we shall see the significance of this extreme readiness 

 shown by its compounds to undergo changes. Eeturn- 



ing from these facts parenthetically introduced, we have next 

 to note that though among the diatomic compounds of the 

 four chief organic elements, there are a few active ones, yet 

 the majority of them display a smaller degree of chemical 

 energy than the average of diatomic compounds. Water is 

 the most neutral of bodies : usually producing little chemical 

 alteration in the substances with which it combines; and 



