Chemistry and Physics. 63 



-4. Elements and Jleta-elements. — In his annual address as Pres- 

 ident of the Chemical Society, Mr. Crookes has continued the 

 discussion upon the nature of the elements which he began in his 

 address at Birmingham. What is the criterion of an element, 

 and how is the line to be drawn between identity and distinct 

 existence? Oxygen, sodium, sulphur, chlorine, are concededly 

 separate elements; and so are chlorine, bromine and iodine. But 

 even here degrees of " elementicity " may be assumed, since chlo- 

 rine is much closer to bromine than it is to oxygen, sodium or 

 sulphur. Nickel and cobalt rank as distinct elements; but 

 would this have been the case had their compounds been identi- 

 ca] in color ? Passing to the rare earth elements, the ground un- 

 der our feet is less secure. Even if scandium, ytterbium, etc., be 

 admitted to the list of elements, what can be said for neo- and 

 praseo-didymium, whose chief chemical claim to individuality is 

 certain slight differences in basicity and crystallizing power, 

 though their physical differences spectroscopically are well 

 marked. If we admit these, how can we exclude the series of 

 bodies discovered by Kriiss and Nilson, or those into which yt- 

 trium, erbium, samarium, etc., have been and are being split up ? 

 Here "the different groupings shade off so imperceptibly the one 

 into the other that it is impossible to erect a definite boundary 

 between any two adjacent bodies and to say that the body on 

 this side of the line is an element, whilst the one on the other 

 side is non-elementary, or merely something which simulates or 

 approximates to an element." " Slight chemical differences, of 

 course, are admitted, and, up to a certain point, so are well- 

 marked physical differences. What are we to say, however, 

 when the only chemical difference is an almost imperceptible ten- 

 dency for the one body — of a couple or of a group — to precipitate 

 before the other? Again, there are cases where the chemical dif- 

 ferences reach the vanishing point, although well-marked physical 

 differences still remain. Here we stumble on a new difficulty : 

 in such obscurities, what is chemical and what is physical ? Are 

 we not entitled to call a slight tendency of a nascent amorphous 

 precipitate to fall down in advance of another a 'physical differ- 

 ence ? ' And may we not call colored reactions depending on, 

 the amount of some particular acid present and varying accord- 

 ing to the concentration of the solution and to the solvent ap- 

 plied 'chemical differences?' I do not see how we can deny 

 elementary character to a body which differs from another by 

 well-marked color- or spectrum reactions, whilst we accord it to 

 another body whose only claim is a very minute difference in ba- 

 sic powers." In answer to the question, how minute differences 

 qualify a substance to be admitted as an element, the author clas- 

 sifies them as follows: (1) Two closely allied substances differing 

 slightly in basic powers and more decidedly in spectrum reac- 

 tions ; such as erbium and holmium, erbium and yttrium; sama- 

 rium and didymium ; these are probably chemical entities. (2) 

 Two substances which, like the white and yellow components of 



