TBANSACTIONS OF SECTION B. 563 



increases with the complexity of molecular structure, and hence he draws the con- 

 clusion that the simpler the molecule the feebler the ahsoi'ption. This conclusion 

 the Abbe Spee regards as perfectly legitimate; but it neither explains nor even 

 necessitates the absence of all absorptive power. 



Granting that helium exists, all analogy points to its atomic weight being 

 below that of hydrogen. Here, then, we may have the very element, with 

 atomic weight half that of hydrogen, required by Mr. Clarke as the basis of 

 Prout's law. . 



But a more important piece of evidence for the compound nature of the chemi- 

 cal elements has yet to be considered. Many chemists must have been struck with 

 certain peculiarities in the occurrence of the elements in the Earth's crust ; it is a 

 stale remark that we do not find them evenly distributed throughout the globe. 

 Nor are they associated in accordance with their specific gravities ; the lighter ele- 

 ments placed on or near the surface, and the heavier ones following serially deeper 

 and deeper. Neither can we trace any distmct relation between local climate and 

 mineral distribution. And by no means can we say that elements are always or 

 chiefly associated in nature in the order of their so-called chemical affinities ; those 

 which have a strong tendency to form with each other definite chemical combina- 

 tions being found together, whilst those which have little or no such tendency exist 

 apart. We certainly find calcium as carbonate and sulphate, sodium as chloride, 

 silver and lead as .sulphides ; but why do we find certain groups of elements with 

 little affinity for each other yet existing iu juxtaposition or commixtiu-e ? The 

 members of some of these groups are far from plentiful, not generally or widely 

 diflused, and certainly they ai'e not easy to separate. 



As instances of such grouping we may mention — 



1. Nickel and cobalt, of which it may be said that had their compounds been 

 coloiu'less they would have been long regarded as identical, and possibly even yet 

 would not have been separated. 



2. The two groups of platinum metals. 



3. The so-caUed 'rare earths,' occurring in gadolinite, samarskite, &c. and 

 evidently becoming more numerous the more closely they are examined. 



Certain questions here suggest themselves : — Is the series of these elements 

 like a staircase or like an inclined plane ? Will they, the more closely they are 

 scrutinised, be found to fade away the more gradually the one into the other P 

 Further, will a mixture hitherto held to be simple, like {e.g.) didymium, be capable 

 of being split up in one direction only, or in several ? I have been led to ask this 

 .last question because I have separated from didymium bodies which seem to agree 

 neither with the praseodymium and neodymium of Dr. Auer von Welsbach, nor 

 with the components detected by M. de Boisbaudran and M. Demar^ay. 



Why, then, are these respective elements so closely associated ? What agency 

 has brought them together ? 



An eminent physicist evades the difficulty by suggesting that their joint occur- 

 rence is simply an instance of the working of the familiar principle ' Birds of a 

 feather flock together.' In their chemical and physical attributes these rare earths 

 are so closely similar that they may be regarded as substantially identical in all 

 the circumstances of solution and precipitation to which they may have been 

 exposed during geological ages. 



But do we, in point of fact, recognise any such agency at work in Nature ? Is 

 there any power which regularly and systematically sorts out the different kinds of 

 matter from promiscuous heaps, conveying like to like and separating unlike from 

 unlike ? I must confess that I fail to trace any such distributive agency, nor, in- 

 deed, do I feel able to form any distinct conception of its natiure. 



I must here remark that coral worms in some cases do effect a separation of 

 certain kinds of matter. Thus a Gorgonia of the species Melithan, and Mussa 

 sinuosa, imdoubtedly eliminate from sea-water not merely lime, but even yttria ; 

 and other recent corals, Pocillojiora dainicornis, and a Symphyllia close to the yttria- 

 secreting Mussa, separate samaria from sea-water. Sea- weeds and aquatic moUusks 

 contain a larger proportion of iodine and bromine than the waters which they 



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