820 KEPORT — 1898. 



of the twist previously imparted to them, rotate the plane of polarisation of electric 

 waves either to the right or to the left. 



If two of the four atoms or groups attached to carbon are identical, there is no 

 asymmetry, and no optical activity. Thus, in a compound of the formula 

 CH.^X'Y', which we may represent by our tetrahedral scheme as shown in lig. 3, 

 the two hydrot^en atoms are equidistant from the carbon atom ; the system has a 

 plane of symmetry passing through X' Y' and the carbon atom, and has therefore 

 a superposable mirror image. 



If the molecule contains only one asymmetric carbon atom, the latter mar 

 be either positive or negative, so that the substance may exist in two forms of 

 opposite optical activity ; in addition to which we may have the racemoid 

 combination of the two, which will be inactive but separable. Mandelic acid, 

 C6H5.CH(OH).COOH,' is a case in point : it is known in these three forms. 



CH(OH).OOOH 



If, as in the case of tartaric acid, | , the molecule contains two 



CH(OH).COOH 

 asymmetric carbon atoms, and at the same time consists of two structurally 

 identical halves, then these two atoms may be either both positive or both nega- 

 tive, reinforcing each other's effect in either case ; or one may be positive and the 

 other negative, when, owing to the structural identity of the two halves of the 

 molecule, the effect of the one will exactly compensate that of the other, and the 

 compound will be inactive, but not separable. Furthermore, there may be the 

 racemic combination of the bi-dextro- form with the bi-loevo- form : a combination 

 inactive, but separable. We have thus the explanation of the four forms observed 

 by Pasteur. 



In fact, all the complex cases of isomerism that have been met with among 

 compounds of this class — compounds structurally identical, but coufiguratively 

 distinct, as it is termed — may be satisfactorily explained, and their possible number 

 accurately predicted, by means of the theory of the asymmetric carbon atom. 



I must apologise to the organic chemists among my audience for inflicting on 

 them this very elementary exposition of what to them is a well-known theory. 

 But outside the circle of organic chemists the theory is. I fear, far from well 

 known. Thus, an eminent physicist, in h's ' Theory of Light,' referring to the 

 rotation of the plane of polarisation by liquid or dissolved substances, says : ' I am 

 not aware that any explanation of it has ever been suggested.' And in the 

 ' Proceedings of the Royal Society ' for the present year, another eminent physicist, 

 after quoting with approval this purely personal confession, goes on to suggest the 

 possibility of the molecules having a twisted structure, and points out that a 

 right-handed twist ' would appear right-handed when looked at from either end,' 

 apparently umiware that such conceptions have been commonplaces of stereo- 

 chemistry for the past quarter of a century at least. 



This brief sketch of the theory was therefore necessary in order that we may 

 now effectively discuss Pasteur's views on the relation between optical activity 

 and life. 



Whenever we prepare artificially, starting either with the elements, or with 

 symmetric cimiiKiunds, any org-anic compound which, when it occurs as a natural 

 product of the living organism, is optically active, the primary product of our 

 laboratory reactions, however closely it may in other respects resemble the natural 

 product, differs from it in being optically inactive. Pasteur was greatly impressed 

 by this fact. In the Lectures deliverediu 1860 he says: ' Artificial products have 

 no molecular asymmetry ; and I could not point out the existence of any more 

 profound distinction between the products formed under the influence of life, and 

 all others.' And again, he refers to ' the molecular asymmetry of natural organic 

 products ' as ' the great characteristic which establishes perhaps the only well- 

 marked line of demarcation that can at present be drawn between the chemistry 

 of dead matter and the chemistry of living matter.' He would not admit that 

 even racemoid forms, optically inactive by inter molecular compensation, might be 



' The asymmetric carbon atom is represented by an italic C. 



