(NO 2 ) 2 O, and in ammonium chloride, NH 4 Cl. If X stand for an atom of hydrogen, or its equivalents, chlorine, hydroxyl, &c., and if O, which is, according to the law of substitution, equivalent to H 2 , be indicated by X 2 , then the three compounds of nitrogen above named should be considered as compounds of the type or form NX 5 . For example, in nitric acid X 5 = O 2 + (OH), where O 2 = X 4 , and OH = X; whilst nitric oxide is a compound of the form NX 2 . Hence this lower form, like lower forms in general, strives by combination to attain to the higher forms proper to the compounds of a given element. NX 2 passes consecutively into NX 3 —namely, into N 2 O 3 and NHO 2 , NX 4 (for instance NO 2 ) and NX 5 .
As the decomposition of nitric oxide begins at temperatures above 900°, many substances burn in it; thus, ignited phosphorus continues to burn in nitric oxide, but sulphur and charcoal are extinguished in it.
This is due to the fact that the heat evolved in the combustion of these two substances is insufficient for the decomposition of the nitric oxide, whilst the heat developed by burning phosphorus suffices to produce this decomposition.
That nitric oxide really supports combustion, owing to its being decomposed by the action of heat, is proved by the fact that strongly ignited charcoal continues to burn in the same nitric oxide70 in which a feebly incandescent piece of charcoal is extinguished.
The compounds of nitrogen with oxygen which we have so far considered may all be prepared from nitric oxide, and may themselves be converted into it. Thus nitric oxide stands in intimate connection with them.71
The passage of nitric oxide into the higher degrees of oxidation and the converse reaction is employed in practice as a means for transferring the oxygen of the air to substances capable of being oxidised. Starting with nitric oxide, it may easily be converted, with the aid of the oxygen of the atmosphere and water, into nitric acid, nitrous anhydride, and nitric peroxide, and by their means employed to oxidise other substances.
In this oxidising action nitric oxide is again formed, and it may again be converted into nitric acid, and so on continuously, if only oxygen and water be present. Hence the fact, which at first appears to be a paradox, that by means of a small quantity of nitric oxide in the presence of oxygen and water it is possible to oxidise an indefinitely large quantity of substances which cannot be directly oxidised either by the action of the atmospheric oxygen or by the action of nitric oxide itself.
The sulphurous anhydride, SO2, which is obtained in the combustion of sulphur and in roasting many metallic sulphides in the air is an example of this kind. In practice this gas is obtained by burning sulphur or iron pyrites, the latter being thereby converted into oxide of iron and sulphurous anhydride. In contact with the oxygen of the atmosphere this gas does not pass into the higher degree of oxidation, sulphuric anhydride, SO3, and if it does form sulphuric acid with water and the oxygen of the atmosphere, SO2 + H2O + O = H2SO4, it does so very slowly.
With nitric acid (and especially with nitrous acid, but not with nitrogen peroxide) and water, sulphurous anhydride, on the contrary, very easily forms sulphuric acid, and especially so when slightly heated (about 40°), the nitric acid (or, better still, nitrous acid) being converted into nitric oxide—
3SO2 + 2NHO3 + 2H2O = 2H2SO4 + 2NO.
The presence of water is absolutely indispensable here, otherwise sulphuric anhydride is formed, which combines with the oxides of nitrogen (nitrous anhydride), forming a crystalline substance containing oxides