TRANSACTIONS OF SECTION B. 821 



artificially prepared ; thus, to the suggestion that the malic acid which he had ob- 

 tained from Dessaigues's artificial aspartic acid might possibly be theracemoid form 

 (as we now know that it is), he replied : ' That is improbable, for then not only should 

 we have made an active body from an inactive one, but we should have made two 

 — a right and a left.' 



The view that racemoids could not be prepared artificially did not long remain 

 tenable. In 1860, the year in which the foregoing lectures were delivered, Perkia 

 and Duppa, and, independently, Kekule, obtained from dibromsuccinic acid a form 

 of tartaric acid which Pasteur recognised as racemic acid. But the succinic acid 

 employed had been prepared from amber, a substance of vegetable origin ; and 

 there was still the possibility that herein lay the source of the optical activity of 

 the two constituents of the artificial racemic acid. This objection, which was 

 raised by Pasteur himself, fell to the ground when, in 187o, Jungfleisch prepared 

 racemic acid from Ma.xwell Simpson's synthetic succinic acid, and separated it 

 into its right and left constituents by means of the sodium ammonium salt. 



' Thus falls the barrier,' wrote Schiitzenberger, ' which M. Pasteur had placed 

 between natural and artificial products. This example shows us how reserved we 

 must be in attempting to draw distinctions between the chemical reactions of the 

 living organism and those of the laboratory.' 



To these words, which, although written a quarter of a century ago, may fairly 

 fee taken as representing the prevailing belief of chemists at the present day, Pas- 

 teur j-eplied as follows : 



' Contrary to M. Schiitzenberger's belief, this barrier still exists. ... To trans- 

 form one inactive compound into miother inactive compound which has the power 

 of resolving itself simultaneously into a right-handed compound and its opposite 

 (son symetrique), is in no way comparable with the possibility of transforming an 

 inactive compound into a single active compozind. This is what no one has ever 

 done ; it is, on the other hand, what living Nature is doing unceasingly before our 

 «ye8.' 



On this and subsequent occasions Pasteur did little more than reiterate opinions 

 which he had previously expressed. As he himself stated, he was then occupied 

 with other problems which absorbed his entire time and energies. The result has 

 been that the opinions have suffered neglect and even misrepresentation. Thus 

 Ostwald, in his ' AUgemeine Chemie,' translating, or rather paraphrasing, the fore- 

 going passage, omits the word ' single ' — which is the key to Pasteur's meaning — 

 and then condemns the statement as illogical. 



Pasteur's point is, that whereas living Nature can make a single optically active 

 compound, those laboratory reactions, to which we resort in synthesising such 

 compounds, always produce, simidtaneously, at least tico, of equal and opposite 

 optical activity ; the result being intermolecular compensation and consequent 

 optical inactivitj\ Not necessarily implied in Pasteur's statement, but entirely in 

 harmony with it, is the fact that we can sometimes produce artificially a single 

 compound containing within its molecule two equal and opposite asymmetric 

 groups, and therefore inactive by iwfr«molecular compensation ; thus in the oxida- 

 tion of maleic acid to mesotartaric acid. 



Let us consider the cause of this limitation of our synthetic reactions. Why 

 cannot we produce, by laboratory processes, involving the play of symmetric forces 

 and the interaction of symmetric atoms and molecules, single optically active com- 

 pounds ? To answer that question, let us turn our attention to the mechanism of 

 the change in which a symmetric carbon atom becomes asymmetric. 



A simple case of such a cha7ige, typical of all similar changes, is the transfor- 

 mation of a compound, CHoX'Y', by substitution, into CHX'Y'Z'. If we follow 

 this process by means of our tetrahedral model, we see at once why, in our ordinary 

 laboratory reactions, both enantiomorphs must be generated in equal quantity. 

 The molecule of the compound, CHOX'Y', of which the tetrahedral representation 

 is given in fig. 3, has, as we have already seen, a plane of symmetry passing 

 through X'Y' and the carbon atom ; and from this plane of symmetry the two 

 hydrogen atoms are equidistant on opposite sides. Any purely mechanical, 



