1892.] on 3Iicro-organ{sms in their Belation to Chemical Change. 629 



being less discriminating, attach both; representatives of a fourth pos- 

 sible class which ivould act upon dulcite but not upon mannite are as yet 

 undiscovered. [Lantern-slide and Plate-culture of B. ethaceticus.^ 



This bacillus, I have recently shown, has the property of breaking 

 down the mannite molecule into alcohol, acetic acid, carbonic 

 anhydride, and hydrogen, but leaves the dulcite molecule untouched. 



More recently I have, in conjunction with my late assistant, 

 Mr. Frew, succeeded in obtaining a micro-organism which decom- 

 poses both mannite and dulcite into alcohol, acetic and succinic acids, 

 carbonic anhydride, and hydrogen. [Lantern-slide and Plate-culture 

 of B. ethacetosuccinicus.^ 



Optically Active Substances. 



But these are by no means the ultimate limits to which the 

 selective or discriminating powers of micro-organisms can be pushed, 

 for although mannite and dulcite are extremely similar substances, 

 they are not chemically identical. We are acquainted, however, 

 with substances which, though chemically identical, are different in 

 respect of certain physical properties, and are hence known as 

 Physical Isomers. It is in explanation of this physical isomerism 

 that one of the most beautiful of chemical theories was propounded 

 by Le Bel and Van't Hoff in 1874, and which remains unsupplanted 

 to the present day. 



This theory depends upon taking into consideration the dissym- 

 metry of the molecule which is occasioned by the presence in it of a 

 carbon-atom which is combined with four different atoms or groups 

 of atoms, and is most easily intelligible] from an inspection of these 

 two models. [Demonstration of tetrahedral models of Asymmetric 

 Carbon-atom.] 



This molecular dissymmetry is specially exhibited in the crystal- 

 line form of such substances, and in their action upon polarised 

 light. 



The molecule arranged according to the one pattern has the 

 property of turning the plane of polarisation in one direction, whilst 

 the molecule arranged according to the other pattern has invariably 

 the property of turning the plane through precisely the same angle 

 in the opposite direction. The molecular dissymmetry ceases when 

 two such molecules combine together, the resulting molecule having 

 no action on polarised light at all. 



The interest of these phenomena in connection with micro- 

 organisms lies in the fact that they are sometimes possessed of the 

 power of discriminating between these physical isomers. Although 

 this remarkable property was demonstrated years ago by Pasteur in 

 respect of the tartaric acids, it has only comparatively rarely been 

 taken advantage of. Eecently, however, chemical science has been 

 enriched in several instances by successfully directing the energies 

 of micro-organisms in such work of discrimination. 



