1887.] on Genesis of the Elements. 49 



emanate from every molecule, but that some of these molecules emit 

 some of these series, others others, and in the jumble of all these 

 molecules, to which is given the name " iodine vapour," the whole 

 seven series are contributors. 



Another important inference to be drawn from the facts is that 

 yttrium atoms, though differing, do not differ continuously, but per 

 saltiim. We have evidence of this in the fact that the spectroscopic 

 bands characteristic of each group are distinct from those of other 

 groups, and do not pass gradually into them. We must accordingly 

 expect, in the present state of science, that this is probably the case 

 with the other elements. And the atoms of a chemical element being 

 known to differ in one respect may differ in other respects, and 

 presumably do somewhat differ in mass. 



Eeturning, after this digression, to the idea of heavy and light 

 atoms, we see how well this hypothesis accords with the new facts 

 here brought to light. From every chemical j^oint of view the stable 

 molecular group, yttrium, behaves as an element. To split up yttrium 

 requires not only enormous time and material, but the existence of a 

 test by means of which the constituents of yttrium are capable of 

 recognition. Had we tests as delicate for the constituent molecular 

 groups of calcium, this element also might be resolved into simpler 

 groupings. It is one thing, however^ to find out means of separating 

 bodies which we know to be distinct and to have colour or spectrum 

 reactions to guide us at every step ; it is quite another thing to 

 separate colourless bodies which are almost identical both in chemical 

 reaction and atomic weight, especially if we have no suspicion that 

 the body we examine is a mixture. 



Again, it seems as if bodies we have been accustomed to regard 

 as absolutely simple and elementary may be split up in different 

 directions according to the means we bring to bear upon them. Until 

 very lately our text-books made mention of an element under the 

 name of didymium. With some trouble it had been separated from 

 its accomjDanying bodies lanthanum and cerium. Its properties had 

 been examined, and no one doubted its distinct and elementary 

 character. It was viewed according to one of the common definitions 

 of an element, as " a something to which we can add, but from which 

 we can take nothing." When, behold! Dr. Auer von Welsbach, 

 examining this supposed simple body in a novel manner, succeeded 

 in decomposing it into two simpler bodies, which he called neodymium 

 and praseodymium ; and later researches, in which I have had a 

 share, show that even neodymium and praseodymium are not the 

 simplest bodies into which didymium can be dissected. 



But it may be asked. What is the bearing of all this upon the 

 great question of the genesis of the elements '? Have we chemists 

 merely discovered some new '• elements," or found out that a body 

 hitherto held to be simple is in reality complex ? We liave, I submit, 

 done something decidedly different. If a metal which is found to 

 have a fixed atomic weight is discovered to be a compound or a 



Vol. XIL (Xo. 81.) e 



