PROGRESS IN CHEMISTRY. 



255 



and so on. It is onl}' necessary to point out in detail the resemblances 

 between the elements which stand in the vertical columns, l>ut it may 

 be stated that the resemblance extends also to the formulae and prop- 

 erties of their compounds. Thus, the chlorides of lithium and sodium 

 are each white, soluble salts of the formulte LiCl and NaCl; oxides of 

 magnesium and of beryllium are both insoluble, white, earthy powders, 

 MgO and BeO (GeO). and so on. Newlands in his preliminary sketch, 

 termed this order the "'Law of Octaves," and predicted the existence 

 of certain undiscovered elements which should occupy unfilled positions 

 in the table. Mendeleef, professor at St. Peters])urg, in 1869, ampli- 

 lied and extended these relations, and he and Meyer pointed out that the 

 volume occupied by equal numbers of atoms of such elements under- 

 went a periodic variation when the elements are classified as above. 

 The prediction of undiscovered elements was made by Mendeleef in a 

 more assured manner, and in several cases they have been realized. 

 Thus, what Mendeleef called ""ekaboron" has since been discovered by 

 Lecoq de Boisbandron and named patriotically "Gallium." Men- 

 deleef s "eka-silicon" is now known as "germanium," discovered b}^ 

 Winkler, and " eka-aluminum " is now Cleve's " scandium." Moreover, 

 the atomic weights of ctiesium, ber^dlium, molyl)denum, and mercury 

 have been altered so that they fit the periodic table, and further 

 research has justified the alteration. 



The valenc}^ of these elements increases from right to left, as will 

 be seen by inspection of the following series : 



NH4CI OH, 



PC3 SO3 



Triad and Pentad. Pyad and Hexad. 



Ne— 



A— 



No valency. 



The elements of no valency are of recent discovery. In 1894 Lord 

 llayleigh had determined the density of the nitrogen of the atmos- 

 phere, having separated from it the oxygen and carbon dioxide which 

 are mixed with nitrogen in air. -He found it to be of somewhat higher 

 density than that obtainable from ammonia and other compounds of 

 nitrogen. In conjunction with Ramsay he investigated atmospheric 

 nitrogen; it was absorbed either by a method devised by Cavendish 

 or by making it combine with magnesium at a red heat. They found 



