50 



SCIENCE 



[N. S. Vol. XLV. No. 1151 



view briefly a few of tlie important dis- 

 coveries that led up to the investigations of 

 Pasteur. These discoveries are outlined in 

 the historical introduction of Pasteur's lec- 

 tures delivered before the Chemical Society 

 of Paris in 1860. 



The fact that a ray of light in passing 

 through a crystal of Iceland spar is divided 

 into two rays had been known for many 

 years, but it remained for Huygens and 

 Newton in 1678 to point out that the ray 

 of light is altered in its properties as it 

 passes through the crystal. Later in 1808 

 Malus was able to show that light was sim- 

 ilarly altered in properties by being re- 

 flected from any object and designated this 

 change in properties by the term "polar- 

 ization." Malus followed up this discov- 

 ery with a number of important observa- 

 tions, but his early death at the age of 37 

 ended his brilliant career. The work, how- 

 ever, was continued by two young phys- 

 icists Arago and Biot, and the latter de- 

 voted a long life primarily to the study of 

 polarized light and its attending phenom- 

 ena. It was Biot who observed that plates 

 cut from certain quartz crystals rotate the 

 plane of polarization to the right while sim- 

 ilar plates cut from other quartz crystals 

 rotate it to the left. In 1815 he announced 

 to the Soeiete philomatique the very impor- 

 tant discovery that this property of rotating 

 the plane of polarization is not confined to 

 solids ; for solutions of certain organic com- 

 pounds, including sugar, camphor and tar- 

 taric acid likewise cause a rotation of the 

 plane. Biot's studies led him to the defi- 

 nite conclusion that while both the quartz 

 crystal and the sugar solution rotate the 

 plane of polarization, the rotation in the 

 two cases is due to different causes. In the 

 former case the crystalline structure must 

 be the influencing factor, for silica in the 

 amorphous state does not rotate the plane 

 of polarization. On the other hand, the 



ability of the sugar solution to rotate the 

 plane of polarization must lie in the struc- 

 ture of the sugar molecule. 



Another important observation in con- 

 nection with this general subject was made 

 by Hauy and Weiss, who noted in their 

 study of quartz crystals that some of the 

 crystals possessed certain faces truncating 

 the upper edge of the prism face. They 

 observed, moreover, that these faces or 

 planes did not occupy the same position 

 on all crystals; that in some, the plane ap- 

 peared on (say) the right upper edge while 

 in others it occupied a similar position on 

 the corresponding left edge. With this 

 difference as a basis, the crystals could be 

 divided into two groups, the members of 

 which are related to each other as object is 

 to image, or as the right hand is to the left. 

 It was Herschel who suggested (1820) that 

 the difference in the optical activities of the 

 quartz crystals, as noted above, might be 

 connected with the position of these trun- 

 cating planes on the crystal, and that of the 

 two groups of crystals referred to, the 

 members of the one group might rotate the 

 plane of polarization in the one direction 

 while the members of the other group might 

 rotate it in the opposite direction. Experi- 

 ments soon proved the verity of Herschel 's 

 suggestion. 



THE INVESTIGATIONS OP PASTEUK BEAEING 

 UPON ASTMMETEY 



The above account gives briefly the state 

 of the knowledge of polarization when Pas- 

 teur began his now classical investigations. 

 In order to familiarize himself with crystals, 

 Pasteur decided to repeat some important 

 research in this field of study — a method 

 which may well be recommended to all who 

 are in training for research work. For this 

 purpose he chose the investigations of de 

 La Provostaye on the tartaric acids pub- 

 lished iu 1841. In this study Pasteur soon 



