1887.] on Genesis of the Elements. 39 



metals of the so-called rare earths. These bodies form but a very 

 trifling portion of the earth's crust. They are chiefly met grouped 

 together in a few minerals, such as samarskite and gadolinite, which, 

 so far, have been found in but few localities, and even in those are far 

 from common. These earths form a group to themselves ; chemically, 

 they are so much alike that it taxes the utmost skill of the chemist to 

 efiect even a partial separation, and their history is so obscure that we 

 do not yet know the number of them. 



It will not be necessary here to explain in detail the process of 

 chemical fractionation adopted for the separation of the rarer earths, 

 since it could interest only the chemical specialist ; moreover, it has 

 been fully described in a paper I read before the British Association 

 at Birmingham. 



Stated in the briefest way the operation consists in fixing upon 

 some chemical reaction in which there is the most likelihood of a 

 difference in the behaviour of the elements under treatment, even 

 though the difierence be slight, and effecting such treatment incom- 

 pletely, so that only a certain fraction of the total bases present is 

 separated : the object being to get part of the material in an insoluble 

 and the remainder in a soluble state. 



Let us suppose that we have in solution two earths almost, 

 identical in their properties, but differing slightly, almost imper- 

 ceptibly, in basicity. We add to the solution of the earths, which 

 must be very dilute, weak ammonia to such an amount only that it 

 precipitates one-half of the bases present. The dilution must be so 

 great that a considerable time must elapse before the liquid shows a 

 turbidity, and several hours will have to pass over before the action 

 of the ammonia is complete. The liquid is then filtered, by which 

 process we have the earths divided into two parts, no longer identical 

 in their composition. We can easily see that there is now a slic^ht 

 difference in the basic value of the two portions of earths ; the portion 

 in solution being, though by a scarcely perceptible amount, more 

 basic than that which the ammonia has precipitated. This minute 

 difference is made to accumulate systematically until it becomes 

 perceptible either by chemical or physical tests. 



The accompanying diagram (Fig. 1), illustrates the scheme of 

 fractionation. Starting from zero at the apex the precipitates all pass 

 to the left and the filtrates to the right. Each circle represents a 

 flask containing the solution under treatment, and the two arrows from 

 each circle show the path pursued respectively by the precipitate and 

 filtrate. 



Such is the general outline of the process. But, as has been 

 already intimated, the methods of separation suitable for different 

 groups of earths vary. Where the constituents of yttrium and 

 samarium are concerned, nothing seems available but straightforward 

 fractionation continued month after month and year after year. 



The further question whether an earth we have separated is really 

 simjile or is still a mixture has again to be decided by yet another 



