yl8 , REPORT— 1898. 



l>y the action of heat Pasteur converted ordinary tartaric acid into racemic 

 acid, in which process a portion of the right acid is converted into the left, an 

 equilibrium being established ; and lievo-tartaric acid may be converted into 

 racemic acid in the same way, the inverse change taking place. At the same time, 

 a new tartaric acid is formed in both cases : mesotartaric acid, or true inactive 

 tartaric acid, which resembles racemic acid in having no action on the plane of 

 polarisation, but diti'ers from it in not being separable into two acids of opposite 

 activity. According to our present views, it contains two equal and opposite 

 asymmetric groups within its molecule. Racemic acid is thus inactive by intev- 

 aiolecular compensation ; mesotartaric acid, by zwfra molecular compensation. 



Pasteur, generalising somewhat hastily from the few cases which he had 

 studied, came to the conclusion that all organic compounds capable of exhibiting 

 optical activity might exist in the ioregoing four forms — dextro, Isevo, racemoid, 

 and meso. As regards the dextro and Isevo forms, this is correct ; as regards the 

 racemiiid form it i.s generally correct ; but the meso form, as we now know, is a 

 very special case, implying that the molecule contains two structurally identical 

 complexes of opposite asymmetry. 



Were I following the exact historical order, I should introduce here Pasteur's 

 view that compounds exhibiting optical activity have never been obtained without 

 the intervention of life^a view which it is the object of the present address to 

 consider. The later developments of stereochemistry, however, throw so much 

 light on this question, and enable us to discuss it with such precision, that we 

 shall turn our attention to these first. Before so doing, however, we may note 

 that, in spite of the immense growth in the material of stereochemistry, and in 

 spite of thp develo]jment of the theoretical views of stereochemists, hardly any 

 experimental method of fundamental importance for the separation and trans- 

 formation of optically active com^iounds has been added to those docribed in 

 Pasteur's classical re>earches, although it is almost forty years since these came to 

 a close. Perhaps Walden's remarkable discovery of a method for the ti-ansforma- 

 tion of certain euHnti'inorphs into their optical opposites without previous racemi- 

 .satioii, is I he only one entitled to be so classed. 



Pasteur was in advance of his time, and his theory of molecular asymmetry 

 was a seed that lay for many years in the ground without germinating. 



In 1858, just about the period when Pasteur was concluding his researches in 

 the foregoing field, Kekule published his celebrated theoretical paper, ' On the 

 Constitution and Metamorphoses of Chemical Compounds, and on the Chemical 

 Nature of Carbon,' in which he showed that, by assuming that the carbon atom 

 had four units ot affinity, the constitution »f orgaftic compounds could be satis- 

 factorily explained. This was the starting-point of the theory of chemical 

 .structure, and from that time to the present day organic chemists have been 

 engaged, with enormous expenditure of labour, in determining the constitution or 

 molecular structure of the carbon compounds on the lines of Kekule's theory. 



In order that Pasteur's ideas should bear fruit it was only necessary that 

 his purely general statements with regard to molecular asymmetry should be 

 specialised, so as to include the recognised constitution of organic compounds. It 

 was from this union of Pasteur's theory with that of Kekul6 that modern stereo- 

 chemistry sprang. The necessary step was taken, independently and almost 

 •simultaneously, by Van't Iloff and Le Bel, in 1874. I will briefly state their 

 conclu.sious, so far as these bear on the subject of optical activity. 



If we examine the structural formulae of a number of thoroughly investigated 

 optically active organic compounds, we shall find that the molecule of each 

 contains at least one carbon atom of which the four affinities are satisfied by 

 four dirterent atoms or groups — an asymmetric carbon atom, as it is termed. 



The four affinities, or directed attractive powers, of the carbon atom are not 

 to be conceived of as lying in one plane. The simplest assumption that we can 

 make with regard to their distribution in space is that the direction of each makes 

 equal angles with the directions of the three others. We may express this 

 differently by saying that the four atoms or groups attached to the carbon atom 

 are situated at the solid angles of a tetrahedron, in the centre of which the carbon 



