The Evolution of the Sciences 



chemistry has enabled an exact description of 

 each body to be provided; this task, which has 

 demanded endless work, is similar to that by 

 which naturalists succeeded in distinguishing and 

 classifying species. It was necessary to sort out 

 in the crowd of characteristics those which were 

 merely accessory: colour, brightness, density, 

 form, so full of importance in the eyes of the 

 older chemist, were recognised to be far from 

 characteristic; if we were guided by them, 

 imagine how many species there would be of 

 carbon, sulphur, selenium and phosphorus. 

 The indisputable criterion, the mark of authen- 

 ticity of each chemical species, had to be found in 

 chemical reaction; we call carbon any body 

 which, combined with oxygen, produces exclu- 

 sively carbonic acid gas. Thus we are on 

 solid ground. In default of this spectrum 

 analysis supplies a criterion of fundamental 

 importance. Every element when rendered 

 luminous by an electric discharge is char- 

 acterised by the emission of bright rays, whose 

 position in the spectrum enable us to recognise 

 and differentiate it; the exquisite delicacy 



of this criterion renders it especially valu- 



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