734 Dr. Thomas Martin Lowry [April 18, 



polarization to the left, and oil of lemon and camphor (dissolved in 

 alcohol) rotating it to the right. 



In the case of quartz, Biot had attributed the rotation of the 

 plane of polarization to the crystalline structure of the material.* 

 The correctness of this view was proved when it was shown that 

 rotatory polarization no longer took place when the crystalline 

 structure of quartz was destroyed by melting it or by dissolving it 

 in alkali. In the case of liquids this explanation was no longer 

 possible. Rotatory polarization must here be attributed to some 

 lack of symmetry in the structure of the molecule rather than of the 

 crystal. It is in such cases that the polarimeter has proved its 

 supreme value in the investigation of molecular structure. In this 

 connexion it will be sufficient if I refer to the classical researches of 

 Pasteur, van't Hoff, and le Bel, and to the brilliant contemporary 

 work of Pope, Kipling, Smiles, and Mills in our own country, and of 

 Meisenheimer and Werner on the Continent. In each of these 

 investigations the development of " optical activity " has been accepted 

 as a conclusive proof of molecular asymmetry, and no firmer basis 

 for theories of molecular structure has yet been found than that 

 which rests upon the use of the polarimeter to detect rotatory 

 polarization. 



I should have liked, if time had permitted, to refer to Faraday's 

 discovery of the rotatory polarization induced in inactive substances, 

 such as glass and water, by exposing them to the influence of a 

 powerful magnetic field. I had also hoped to be able to demonstrate 

 this phenomenon in one of the pieces of heavy glass prepared by 

 Faraday himself, using for this purpose the large electro-magnet 

 employed by the late Sir William Perkin in his investigations. 

 But a passing reference to this third method of producing rotatory 

 polarization is all that is possible to-night. 



C. — MUTAEOTATION. 



In 1846, thirty years after Biot had discovered that rotatory 

 polarization might occur in liquids as well as in crystals, a remark- 

 able discovery was made by the French chemist Dubrunfaut in 

 reference to the rotatory power of aqueous solutions of grape-sugar 

 or glucose. Dubrunfaut found that by using freshly-prepared 

 solutions of the sugar he could observe a transient rotatory power 

 which was twice as great as that observed in solutions which had 

 been prepared a few hours previously. To this remarkable 

 phenomenon he gave the name Birotation. 



The same phenomenon, which is now generally known as Muta- 

 rotation, has since been observed in the case of nearly all the 



* In associating rotatory polarization with double refraction, he was wrong, 

 as crystals of sodium chlorate, which show no double refraction, are still capable 

 of rotating the plane of polarized light. 



