The Evolution of the Sciences 



constants, such as colour, density, solubility in 

 solvents, co-efficients of expansion and of 

 elasticity, specific heat are variable; some- 

 times even their chemical properties are modified. 

 Thus, ozone differs distinctly from oxygen; car- 

 bon, sulphur, selenium, and phosphorus can exist 

 under very different forms with very different 

 properties. This variability of species is of a 

 nature to suggest the idea of a chemical evolution 

 analogous to biological evolution. It is of course 

 possible to attribute these varieties to diffeient 

 states of molecular aggregation ; the molecule of 

 ozone might contain three atoms, whereas the 

 molecule of oxygen contains only two; this 

 would protect piovisionally, and thanks to a 

 plausible hypothesis, the individuality of the 

 elements. But there are other cases in which 

 this kind of argument does not apply. 



Let us consider for example the oxides whose 

 metals form what are usually called the rare 

 earths. These bodies were, in fact, rare and 

 imperfectly known until the increased efforts to 

 obtain them due to the requirements of modern 

 industry showed their relative abundance and 

 in any case their wide dissemination in the 



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