64 Scientific Intelligence. 



cerium, have no distinct spectrum reaction and differ in basicity 

 so slightly that their separation has not been possible, and yet 

 have differently colored oxides; these the author would concede 

 to be elemeuts. (3) Two substances, obtained from different 

 minerals, which have no recognizable chemical differences, but 

 the phosphorescent spectrum of one having a strong line which is 

 absent in that of the other; as the yttria from samarskite and 

 that from gadolinite. (4) An earth like yttria or samaria having 

 a definite phosphorescent spectrum, and which is separable only 

 with enormous difficulty from its associates, but to which the ad- 

 dition of another substance greatly intensifies one or more of the 

 lines of its spectrum, the others remaining unaffected. Is such 

 an earth simple or compound? (5) An earth which, like calcium 

 sulphate, showing no difference on fractionation, has a phosphor- 

 escent spectrum not materially modified by admixture, but the 

 residual glow of one part of which is suppressed as seen in the 

 phosphoroscope, while the rest is unaffected. (6) Earths, like 

 yttrium obtained from different minerals, which behave alike 

 "both chemically and spectroscopically, except that in the spec- 

 trum of the one a certain line is a- little brighter than it is in that 

 of the other. The difficulty arising from the periodic theory, 

 Mr. Crookes obviates by suggesting the use of the term, " ele- 

 mentary group" for "element." "Let us say, e. g., the smallest 

 ponderable quantity of yttrium is an assemblage of ultimate 

 atoms almost infinitely more like each other than they are like 

 the atoms of any other approximating element. It does not nec- 

 essarily follow that the atoms shall all be absolutely alike among 

 themselves. The atomic weight which we ascribe to yttrium 

 therefore merely represents a mean value around which the actual 

 weights of the individual atoms of the 'element ' range within 

 certain limits. But if my conjecture is tenable, could we separate 

 atom from atom we should find them varying within narrow limits 

 on each side of the mean." Considering the process of fractional 

 precipitation, as when to a dilute solution of yttrium, dilute am- 

 monia is added in quantity sufficient to precipitate only half of 

 the yttria, "the problem is not why a precipitate is produced, 

 but what determines or directs some atoms to fall down and oth- 

 ers to remain in solution. Out of the multitude of atoms present, 

 what power is it that directs each atom to choose the proper 

 path ? TVe may picture to ourselves some directive force passing 

 the atoms one by one in review, selecting one for precipitation 

 and another for solution till all have been adjusted. In order 

 that such a selection can be effected there evidently must be 

 some slight differences between which it is possible to select, and 

 this difference almost certainly must be one of basicity, so slight 

 as to be imperceptible by any test at present known, but suscept- 

 ible of being nursed and encouraged to a point where the differ- 

 ence cam be appreciated by ordinary tests." To make clearer 

 this deviation from absolute homogeneity, the author goes back 

 to the earliest dawn of the material universe, finds " an infinite 



