570 PRINCIPLES OF CHEMISTRY 



metals form compounds which are, in all respects, stable, and in which 

 the original characters of the halogens and alkali metals have entirely 

 disappeared. The formation of such compounds is accompanied by 

 evolution of a large amount of heat, and by an entire change of both 

 the physical and chemical properties of the substances originally taken. 

 The alloy of sodium and potassium, although liquid at the ordinary 

 temperature, is perfectly metallic, like both its components. The 

 compound of sodium and chlorine has neither the appearance nor the 

 properties of the original elements ; sodium chloride melts at a higher 

 temperature, and is more difficultly volatile, than either sodium or 

 chlorine. 



With all these qualitative differences there is, however, an important 

 quantitative resemblance between the halogens and the alkali metals. 

 This resemblance is clearly expressed by stating that both orders of 

 elements belong to those which are univalent with respect to hydrogen. 

 It is thus possible to express that both the above-named orders of ele- 

 ments replace hydrogen atom .for atom. Chlorine is able to take the 

 place of hydrogen by metalepsis, and the alkali metals take the place 

 of hydrogen in water and acids. As it is possible to consecutively re- 

 place every equivalent of hydrogen in a hydrocarbon by chlorine, so it 

 is possible in an acid containing several equivalents of hydrogen to 

 replace the hydrogen consecutively equivalent after equivalent by an 

 alkali metal ; hence an atom of these elements is analogous to an 

 atom of hydrogen, which is taken, in all respects, as the unit for the 

 comparison of the other elements. In ammonia and in water chlorine 

 and sodium are able to bring about a direct replacement. According to 

 the law of substitution, the formation of sodium chloride, NaCl, already 

 shows the equivalence of the atoms of the alkali metals and the halo- 

 gens. The halogens and hydrogen and the alkali metals combine with 

 such elements as oxygen, and it is easily seen that in such compounds 

 one atom of oxygen is able to retain two atoms of the halogens, of 

 hydrogen, and of the alkali metals. In this respect it is enough to 

 compare the compounds KHO, K 2 O, HC1O, and C1 2 O, with water. It 

 must not be forgotten, however, that with oxygen the halogens give 

 higher acid grades of oxidation, besides compounds of the type R 2 O> 

 which the alkali metals and hydrogen are not capable of forming. We 

 shall soon see that these relations are also subject to a special law, 

 showing the gradual transition of the properties of the elements from 

 the alkali metals to the halogens. 43 



45 We may here observe that the halogens, and especially iodine, may play the part 

 of metals (hence iodine is more easily replaced by metals than the other halogens, and it 

 approaches nearer to the metals in its physical properties than the other halogens). 



