1102 



80 90 100 110 120 130 140 150 160 170 180 ^^^^j^ ^^^^^^^ 



And even if this is not always the case, it must he clear, that 

 with chemically not (juite homogeneous material, there will be 

 found (luantitative data, from which must follow other relations 

 between different properties, as they would follow from observations 

 with absolutely pure substances. 



Moreover the available data considering specific weights of corre- 

 sponding derivatives of the rare earth-metals, are highly rudimentary 

 and not very trustworthy, as will be shown later in the fall paper 

 on these investigations. With respect to the available data it can 

 liardly be justified to suppose a general parallelism between atomic 

 weights and atomic volumes within this group of homologous elements. 



I may remind readers of the fact, that in a somewhat different 

 train of thought Mr. Urbain has shown just such a kind of periodicity 

 to exist with his spectroscopically pure oxides, with respect to some 

 other properties. He was able to show '), that the coefficients of 

 magnetisation n of the oxides of these metals, determined after the 

 method of Curie and Cheneveau, and compared with the value of 

 ft of pure cobaltsulphate: CoSO,, 1 H^O, were (X^O-c): 



for: xV(/ Sm Eu Gd Tb Dys 



33,5 6,5 33,5 J 61 237 290 



while these numbers are decreasing continually, passing liolmium with 

 a not yet known, but probably very great value of n, over erbium, 



1) Urbain et Jantsgh, Compt. rend. 174. 1286. (1008). 



