478 THE POPULAR SCIENCE MONTHLY. 



and certain of the salts of the alkaline-earth metals, calcium, strontium, 

 and barium, being very readily volatile, upon heating these salts, in 

 the non-luminous flame of a Bunsen gas-burner for example, they un- 

 dergo vaporization, and their vapors become incandescent and capable 

 of yielding the characteristic emission spectra of the pai'ticular metals. 

 In examining in this way the alkali-salt residue of a mineral water 

 from Durkheim, Bunsen observed in the spectrum before him certain 

 colored lines not belonging to any one of the then known alkalies, 

 potash, soda, or lithia ; and yet necessarily belonging to some sub- 

 stance having the general characters of an alkali, since all other bodies 

 than alkalies had been previously removed from the residue under ex- 

 amination. In full reliance upon the certainty of this conclusion, Bun- 

 sen evaporated some forty tons of the water in question ; and from the 

 alkali-salt residue succeeded in extracting and separating salts of two 

 new alkali-metals, each characterized by a well-marked pair of lines in 

 the blue or indigo, and one of them having in addition a pair of well- 

 marked lines of extremely small refrangibility in the red of the spec- 

 trum. From its yielding those red lines, the one metal was named 

 rubidium ; the other, of which the bright-blue lines were especially 

 characteristic, being called caesium. 



The very general distribution in Nature of these two elements was 

 speedily established, and salts of each of them were, with much labor, 

 eventually prepared in a state of purity and in reasonable quantity. 

 From certain of their respective salts the metals themselves were ob- 

 tained by the usual processes, and, together with their salts, were sub- 

 mitted to detailed chemical examination. And no sooner was this ex- 

 amination made, than the position of the newly-discovered elements, as 

 members of the alkali-metal family, at once became apparent. Rubid- 

 ium and caesium were found in all their properties to present the most 

 striking analogy to potassium, and evidently to stand to this metal in 

 the same relation that strontium and barium respectively stand to cal- 

 cium ; while they differed from sodium, much as strontium and barium 

 respectively differ from magnesium. This relationship in obvious 

 properties was further borne out by the relationship of their atomic 

 weights, thus : 



Mg 24 .. .. Na 23 



Ca 40 . . . . l K 39 



Sr 87 . . . . \ Rb 85 



Ba 137 . . . . { Cs 133 



F 19 .... O 16 



CI 35.5 . . . . ( S 32 

 Br 80 . . . . \ Se 79 

 I 127 .. .. I Tel29 



It is observable that the sequence of atomic weight in the thus com- 

 pleted alkali-metal family is strictly parallel to the previously well- 

 known sequences in the alkali-earth metal family, and in the halogen 

 and oxygen families respectively. Moreover, just as the basylity of 

 the alkaline-earth metals increases in the order of their several atomic 

 weights calcium being less basylous than strontium, and far less basy- 

 lous than barium so also is the basylity of potassium inferior to that 



