462 PRINCIPLES OF CHEMISTRY 



water, we need not multiply instances, but must turn our attention to two > 

 special circumstances which are closely connected with the very mechanism, 

 of substitutions. 



In the first place, the replacement of two ; atoms of hydrogen by one atom 

 of oxygen may take place in two ways, because the hydrogen molecule is 

 composed of two atoms, and therefore, under the influence of oxygen, the 

 molecule forming water may separate before the o. gen has time to take its 

 place. It is for this reason that we find, during the conversion of alcohol 

 into acetic acid, that there is an interval during which is formed aldehyde* 

 C 2 H 4 0, which, as its very name implies, is alcohol dehydrogenatum,' or 

 alcohol deprived of hydrogen. Hence aldehyde combined with hydrogen 

 yields alcohol ; and united to oxygen, acetic acid. 



For the same reason there should be, and there actually are, intermediate 

 products between ammonia and nitric acid, N0 8 (HO), containing either less 

 hydrogen than ammonia, less oxygen than nitric acid, or less water than 

 caustic ammonia. Accordingly we find, among the products" of the deoxida- 

 tion of nitric acid and the oxidation of ammonia, not only hydroxylamine, 

 but also nitrous oxide, nitrous and nitric anhydrides. Thus, the production 

 of nitrous acid results from the removal of two a'toms of hydrogen from 

 caustic ammonia and the substitution .of the oxygen for the hydrogen, 

 NO(OH) ; or by the substitution, in ammonia, of three atoms of hydrogen by 

 hydroxyl, N(OH) 3 , and by the removal of water: N(OH) 3 -H i O = NO(OH). 

 The peculiarities and properties of nitrous acid as, for instance, its action on 

 ammonia and its conversion, by oxidation, into nitric acid are thus clearly 

 revealed 



On the other hand, in speaking of the principle of substitution as applied 

 to water, it is necessary to observe that hydrogen and hydroxyl, H and OH, 

 are not. only competent to unite, but also to form combinations with them- 

 selves, and thus become H^ and Hj0. 2 ; and such are hydrogen and the 

 peroxide thereof. In general, if a molecule A B exists, then molecules A A 

 and BB can exist also. A direct reaction of this kind does not, however, 

 take place in water, therefore undoubtedly, at the moment of formation, 

 hydrogen reacts on hydrogen peroxide, as we can show at once by 

 experiment; and further because hydrogen peroxide, HjOa, exhibits a 

 -structure containing a molecule of hydrogen, H 2 , and one of oxygen, 2 , 

 ither of which is capable of separate existence. The fact, however, may 

 now be taken as thoroughly established, that, at the moment of combustion 

 of hydrogen or of the hydrogen compounds, hydrogen peroxide, is always 

 formed, and not only so, but hi all probability its formation invariably pre- 

 cedes the formation of water. This was to be expected as a consequence of 

 the law of Avogadro and Gerhardt, which leads us to expect this sequence 

 in the case of equal interactions of volumes of vapours and gases ; and in 

 hydrogen peroxide we actually have such equal volumes of the elementary 



The. instability of hydrogen peroxide that is to say, the ease with 

 which it decomposes into water and oxygen, even at the mere contact of 

 porous substances accounts for the circumstance that it does not form a per- 

 manent product of combustion, and is not produced during the decomposition 



