Natural Classification of Elementary Substances. 471 



between the spectra of the odd and even series, taken hori- 

 zontally, that the spectra of members of the third series shift 

 in the same relation to the second as the spectra of Li, Na, 

 K, Rb, Cs, i. e. to the less refrangible side of the spectrum 

 with increasing atomic weights. The same inversions of dis- 

 placement are also observable in the spectra of the odd and 

 even members of the vertical series Hn, H2n, H3n as are 

 found in the horizontal series ; the spectra of the heavy 

 elements, like Cu, Ag; Zn, Cd, shifting to the more refran- 

 gible side of the spectrum in relation to the alkaline and 

 alkaline -earth metals on each side of them. 



The spectra of members of the highest series of elements, 

 both vertically and horizontally, shift, on the whole, towards 

 the less refrangible side of the spectrum with increase of 

 atomic weights, like the spectra of the alkaline metals. 



Mendeleetf and others have pointed out the greater re- 

 semblance of chemical and other properties which odd or 

 even series of elements have to each other than to the imme- 

 diately adjoining series. Hence the spectra of Gra, In, Tl, 

 in the third series, have a greater resemblance to the spectra 

 of their homologues of position, K, Rb, Cs, in the first series, 

 than they have to their homologues Ca, Sr, Ba, in the second 

 (even) series. The chemical and other properties of hydro- 

 gen and the two constituents of reputed helium (H, H2, Ho) 

 may therefore be expected to stand in the same order to each 

 other as their homologues of position in the first, second, and 

 third vertical series of elements, Hn, H.2n, TL3n. 



Profs. Runge and Paschen, in their endeavour to bring 

 the two new gases into a classification in accordance with the 

 requirements of MendeleefPs so-called periodic law, have 

 placed them in the first series between hydrogen and lithium, 

 notwithstanding that they, at the same time, show that the 

 spectra of these gases shift in the opposite direction to the 

 spectra of the alkalies Li, Na, K, Rb, Cs. 



The German physicists can hardly have realized the con- 

 sequences to MendeleefPs system by placing the two gases 

 between hydrogen and lithium. In the paper which I read 

 before the Society in December last I gave a demonstration 

 of the confusion that would be brought into the so-called 

 periodic system by the discovery of a new element x in the 

 particular series where these physicists propose to place the 

 new gases*. There is absolutely no place in MendeleefFs 

 system for elements with atomic weights between lithium 

 and hydrogen, as the Russian chemist never contemplated 

 the existence of elements with properties and cosmical rela- 

 tions such as the new gases have been found to possess. 

 * Memoirs Manchester Lit. and Phil. Society, vol. ix. p. 77. 



