POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIf.M 569 



specific gravities of T52 and 1-88 respectively, and melt at 39 and 

 27. 



Judging by the properties of the free metals, and of their corre- 

 sponding and even very complex compounds, lithium, sodium, potas- 

 sium, rubidium, and caesium present an indubitable chemical resem- 

 blance. The fact that the metals easily decompose water, and that 

 their hydroxides RHO and carbonates R 2 CO 3 are soluble in water, 

 whilst the hydroxides and carbonates of nearly all other metals are 

 insoluble, confirms this, and if the resemblance between the correspond- 

 ing salts be taken into consideration there is no doubt that the resem- 

 blance in the chemical character of these metals is very considerable : 

 therefore they form a natural group of alkali metals. The halogens 

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

 the elements. Some of the other elements are metals approaching in a 

 certain degree 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, and are displaced by the latter from the majority of their com- 

 pounds ; they also evolve less heat in combining with the halogens, and 

 form less energetic bases than the alkali metals. Such are the common 

 metals, silver, iron, copper, <tc. Some other elements, in the character 

 of their compounds, 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 defined in them than 

 in the halogens themselves. Sulphur, phosphorus, arsenic, &c., belong 

 to this order of elements. Lastly, there is yet another order of elements, 

 like carbon and nitrogen, in which neither the metallic nor the halogen 

 properties are clearly defined, and which in this respect occupy an 

 intermediate position between the two above-mentioned orders of ele- 

 ments. 



The clear 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 acid 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 compo- 

 nents. On the other hand, the halogens on combining with the alkali 



