496 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



C. ILLUSTRATIONS AND APPLICATIONS OF THE PERIODIC 



LAW. 



The periodic law was illustrated by L. Meyer (1869) in refer- 

 ence to the valency and the atomic volumes of the elements ; 

 the former property justifies the classification into octaves or 

 short periods, the latter property proves the existence of five 

 main periods and two fragments. Dewar (1913) has shown 

 that the atomic heats at low tempero-tures exhibit the same 

 periodic variations as the atomic volumes. Other periodic 

 properties are melting-point, compressibility, heat of formation 

 of oxides and chlorides, etc. 



The periodic system has been used to correct certain atomic 

 weights, in which there was doubt as to the equivalent, as in the 

 case vigold, or the valency, as in the cases of beryllium, indium 

 and uranium ; but the atomic weights of argon and potassium, 

 cobalt and nickel, tellurium and iodine do not conform to the 

 order suggested by the periodic classification. Mendeleeff also 

 used the periodic classification to predict the properties of 

 missing elements, of which three have since been discovered, 

 thus 



Rka-boron Rka-aluminium Eka-silicon 



= Scandium, 44*1 = Gallium, 69*9 = Germanium, 72*5 



(Nilson, 1879) (Lecoq de Boisbaudran, 1875) (Winkler, 1886) 



His predictions in the range covered by the rare-earth elements 

 were incorrect, but his eka-manganese= 100 and tri-manganese 

 = 190 are still regarded as vacant places in the periodic classifi- 

 cation. 



D. ATOMIC WEIGHTS AND ATOMIC NUMBERS. 



Prout, in 1815-1816, suggested that the atomic weights of the 

 elements were integral multiples of the atomic weight of hydro- 

 gen. " Prout's hypothesis " has not been confirmed by experi- 

 ment, but Strutt has shown that the tendency of the atomic 

 weights to approximate to whole numbers is greater than can 

 be accounted for by the law of probability in the ratio 

 of 1000 : i. 



Newlands, in 1864, attached to the elements a series of 

 consecutive atomic numbers, and thereby discovered the law 



