COMI'ofNDS ()!' MTKor.KN WITH HYDROGEN AND <>XY<;KN 257 



pounds the most stable should be chosen. We will, therefore, take 

 water as the most stable compound of hydrogen and oxygen, 24 in order 

 to see the cases of substitution between hydrogen and oxygen ; but 

 for the sake of clearness we will start by taking the very stable mole- 

 cule of hydrochloric acid, HC1, as one which can be divided into H 

 and Cl only. According to the law of substitution, if these elements 

 are able to form a molecule, and a stable one, then they are able to 

 replace each other. And, indeed, we shall afterwards see that in a 

 number of instances a substitution between hydrogen and chlorine 

 conversely takes place. Given RH, then RC1 is possible, because 

 HC1 exists and is stable. The molecule of water, H 2 0, may be 

 divided in two ways, because it contains 3 atoms : into H and (HO) 

 on the one hand, and into H 2 and O on the other. Consequently, being 

 given RH, its substitution products will be R(HO) according to the 

 first form, and R 2 O according to the second ; being given RH 2 , its 

 corresponding substitution products will be RH(OH), R(OH) 2 , RO, 

 (RH). 2 O, c. The group (OH) is the same hydroxyl or aqueous 

 radicle which we have already mentioned in the third chapter as a 

 component part of hydroxides and alkalis for instance, Na(OH), 

 Ca(OH) 2 , Ac. It is evident, judging from HC1, that (OH) can be 

 substituted by Cl, because both are replaceable by H; and this is of 

 common occurrence in chemistry, because metallic chlorides for in- 

 stance, NaCl and NH 4 C1 correspond with hydroxides of the alkalis 

 Xa(OH) or NH 4 (OH). In hydrocarbons for instance, C 2 H 6 the 

 hydrogen is replaceable by chlorine and by hydroxyl. Thus common 

 alcohol is C 2 H 6 , in which one atom of H is replaced by (OH) ; that 

 is, C 2 H 5 (OH). It is evident that the replacement of hydrogen by 

 hydroxyl essentially forms the phenomenon of oxidation, because RH 

 gives R(OH), or RHO. Hydrogen peroxide may in this sense be 

 regarded as water in which the hydrogen is replaced by hydroxyl ; 

 H(OH) gives (OH) 2 or H 2 O 2 . For this reason chlorine, as we shall 

 afterwards see, exhibits in its reactions much analogy to hydrogen 

 peroxide, which may be termed free hydroxyl. The other form of 

 substitution namely, that of O in the place of H 2 is also a common 

 chemical phenomenon. Thus common alcohol, C. 2 H 6 O, or C 2 H 5 (OH), 

 when oxidising in the air, gives, as every one knows, acetic acid, 

 C 2 H 4 O 2 , or C 2 H 3 0(OH), in which H 2 is replaced by O. 



2 * If hydrogen peroxide be taken as a starting point, then still higher forms of oxi- 

 dation than those corresponding with water should be looked for. They should possess 

 the properties of hydrogen peroxide, especially that of parting with their oxygen with 

 extreme ease (even by contact). Such compounds are known. Pernitric, persulphuric, 

 and similar acids present these properties, as we shall see in describing them. 



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