46 A. W. Stewart on 



this result is compared with Meiideleef's table it is seen that 

 blanks were actually left by Mendeleef at these very points. 

 Thus two lines of evidence, chemical and physical, indicate that 

 there are still five elements to be discovered. 



In support of such forecasts we know that in 1871 scandium, 

 gallium and germanium were unknown, but Mendeleef, from the 

 gaps in his table, predicted the fact of their existence and even 

 their very properties with amazing certitude. 



Table III. 



Prediction (1871). Properties of (jlermanium (1886). 



Atomic Weight r= 72. Atomic Weight = 72-5. 



Grey Metal, hard to fuse. Grey Metal, fuses 900° c. 



Specific Gravity =5-5. Sp. G. =5-469. 



Density of Oxide =4-7. Density of Oxide =4-703. 



Volatile liquid Chloride, boiling Chloride is a volatile liquid, 

 at 90°C. with density of 1 -9 boiling at 86°C. density 1 -887. 

 Easily forms volatile Fluoride. Fluoride volatile. 



Thus from consideration of his periodic table Mendeleef was able 

 to predicate the existence of an unknown element and even to 

 forecast its properties with wonderful accuracy. He was equally 

 successful with the other two elements, scandium and gallium. 



These examples show the extreme importance of the 

 classification in the Periodic Table. The actual causes of these 

 variations are not yet known, but in the future we may learn 

 something of the laws which have been thus revealed. 



[To indicate the resemblances between members of the same 

 group of elements many experiments were shown, among them 

 vacuum tubes of the rare gases, tints imparted to a Bunsen fiame 

 by the presence of lithium and its congeners, while other 

 resemblances were illustrated by the formation and non-formation 

 of precipitates on the addition of certain reagents.] 



