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application of the exact methods of physical and 

 chemical research to problems which had hitherto been 

 attacked by other less precise and less systematic 

 methods. His early researches were of a purely 

 chemical nature. It is now sixty years since he pub- 

 lished his first investigation. But this pointed out 

 the character of the man and indicated the lines upon 

 which all his subsequent work was laid. 



Of all the marvellous and far-reaching discoveries of 

 modern chemistry, perhaps the most interesting and 

 important is that of the existence of compounds which 

 whilst possessing an identical composition, that is, 

 made up of the same elements in the same proportions 

 are absolutely different substances as judged by 

 their properties. The first instance made known to us 

 of such isomeric bodies, as they are termed by the 

 chemist, was that pointed out by Berzelius. He 

 showed that the tartaric acid of the wine-lees possesses 

 precisely the same composition as a rare acid having 

 quite different properties which is occasionally found 

 in the tartar deposited from wine grown in certain 

 districts of the Vosges. Berzelius simply noted this 

 singular fact, and did not attempt to explain it. Later 

 on, the celebrated French physicist Biot observed that 

 not only do those two acids differ in their chemical 

 behaviour but likewise in their physical properties, 

 inasmuch as the one (the common acid) possesses the 

 power of deviating the plane of a polarised ray of 

 light to the right, whereas the rare acid has no such 

 rotatory power. It was reserved, however, for Pasteur 

 to give the explanation of this curious and at that 

 time unique phenomenon, for he proved that the 

 optically inactive acid is made up of two compounds, 

 each possessing the same composition, but differing in 

 optical properties. The application of the same method 



