578 PEINCIPLES OF CHEMISTRY 



hydroxides &HO and carbonates R 2 C0 3 are soluble in water, whilst the 

 hydroxides and carbonates of nearly all other metals are insoluble, shows 

 that these metals form a natural group of alkali metals. The halogens 

 and the alkali metals form, by their character, the two extremes of the 

 elements. Many of the other elements are metals approaching the 

 alkali metals, both in their capacity of forming salts and in not forming 

 acid compounds, but are not so energetic as the alkali metals, that is, 

 they form less energetic bases. Such are the common metals, silver, 

 iron, copper, &c. Some other elements, in the character of their com- 

 pounds, approach the halogens, and, like them, combine with hydrogen, 

 but these compounds do not show the energetic property of the halogen 

 acids ; in a free state they easily combine with metals, but they do not 

 then form such saline compounds as the halogens do in a word, the 

 halogen properties are less sharply denned in them than in the halogens 

 themselves. Sulphur, phosphorus, arsenic, &c. belong to this order 

 of elements. The clearest distinction of the properties of the halogens 

 and alkali metals is expressed in the fact that the former give acids 

 and do not form bases, whilst the latter, on the contrary, only give 

 bases. The first are true add elements, the latter clearly-defined basic 

 or metallic elements. On combining together, the halogens form, in a 

 chemical sense, unstable compounds, and the alkali metals alloys in 

 which the character of the metals remains unaltered, just as in the 

 compound IC1 the character of the halogens remains undisguised ; thus 

 both classes of elements on combining with members of their own class 

 form non -characteristic compounds, which have the properties of their 

 components. On the other hand, the halogens on combining with the 

 alkali 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 correct to say that both the above-named orders of ele- 



