TRANSACTIOXS OF SECTION B. 817 



the atoms of the left acid realise precisely the asymmetric grouping which is the 

 inverse of this.' 



The idea of the irregular tetrahedron is, it may he explained, derived from the 

 hemihedral facets. Imagine these to develop in the case of dextro-tartaric acid 

 until the other faces of the crystal disappear, and there results an irregular tetra- 

 hedron. Repeat the process with a crystal of laevo-tartaric acid, and the enantio- 

 morphous tetrahedron — the mirror-image of the former — is ohtained. We shall 

 see later that the idea, on the one hand, of two asymmetric tetrahedra, and, on 

 the other, that of two opposite hehces, given as alternatives hy Pasteur to explain 

 the grouping of the atoms within the molecules of dextro- and loevo-tartaric acids, 

 are in reality identical. 



The precision of Pasteur's views as to the asymmetry of these acids enabled 

 him to discover two further methods of separating them. Thiis he points out that 

 although these acids will possess equal aiSnity for any given symmetric base, such 

 as potash, or ammonia, or aniline, yet their affinities will not be equal if the base, 

 like quinine or strychnine, is itself asymmetric ; because here the special one-sided 

 asymmetry of the base will modify its mode of combination with the two enantio- 

 morphous acids. The solubility is different in the case of the dextro- and Isevo- 

 tartrates of the same asymmetric base ; the crystalline form, the specific gravity, 

 the number of molecules of water of crystallisation, may be all different. Potassium 

 dextro- and Isevo-tartrates are mirror images of one another ; quinine dextro- and 

 loevo-tartrates are not. Pasteur employed in his experiments the asymmetric base 

 cinchonicine, which he converted into its acid racemate, and allowed the solution 

 to crystallise. The first crystallisations consisted of pure IjBvo-tartrate of cin- 

 chonicine, whilst the more soluble dextro-tartrate remained in the mother liquor, 

 from which it finally crystallised in forms totally distinct from those of the loevo- 

 tartrate. 



Pasteur's third method is of physiological interest, and is, moreover, the 

 stepping-stone to his later work on ferments. As we shall see presently, he 

 regarded the formation of asymmetric organic compounds as the special prerogative 

 of the living organism. Most of the substances of which the animal and vegetable 

 tissues are built up — the proteids, cellulose — are asymmetric organic compounds, 

 displaying optical activity. Pasteur had shown that two compounds of inverse 

 asymmetry behaved differently towards a third asymmetric compound. How 

 would they behave towards the asymmetric living organism ? 



It had frequently been noticed that impure calcium tartrate, when mixed with 

 organic matters, as is the case when it is obtained in the process of preparing 

 tartaric acid from argol, readily underwent fermentation. Pasteur examined the 

 action of the ferment (apparently a Penicillium) on ammonium tartrate — a sub- 

 stance which had the advantage over calcium tartrate of being soluble — and, finding 

 that the fermentation here followed a normal course, ending with the destruction 

 of the tartrate, repeated the experiment with ammonium racemate, examining the 

 solution from time to time with the polarimeter. The fermentation proceeded^ 

 apparently, as before ; but the solution, originally optically inactive, became Isevo- 

 rotatory, the activity gradually increasing in amount until a maximum was 

 reached. At this point the fermentation ceased. The whole of the dextro- 

 tartrate had disappeared, and from the solution the Itevo-tartrate was obtained in 

 a state of purity. The asymmetric living organism had selected for its nutriment 

 that particular asymmetric form of tartaric acid which suited its needs — the form, 

 doubtless, which in some way fitted its own asymmetry — and had left the opposite 

 form either wholly or, for the most part, untouched. The asymmetric micro- 

 organism, therefore, exhibits a power which no symmetric chemical substance, 

 such as our ordinary oxidising agents, and no symmetric form of energy, such as 

 heat, can ever possess : it distinguishes between enantiomorphs. If we oxidise 

 racemic acid with nitric acid, for example, both the enantiomorphous constituents 

 are attacked in exactly the same degree. If we hent racemic acid, whatever 

 happens to its right-handed constituent happens equally to its left-handed con- 

 stituent: the temperature of decomposition of both is the same. Asymmetric 

 agents can alone display selective action in dealing with enantiomorphs. 



1898. 3 G 



