Supplement to "Nature" December 23, 1922 



year 1S19 should have made his celebrated discovery 

 of isomorphism during his first research, that Scheele 

 in 1769 should have isolated tartaric acid as the result 

 of his first investigation, and that Pasteur in 1848 

 should have discovered the great principle of enantio- 

 morphism and the generalisation connecting it with 

 optical activity now known in crystallography as 

 Pasteur's Law, as the result of his earliest research. 

 Young post-graduates setting forth on their first steps 

 in scientific research may find great encouragement 

 from these interesting facts. 



In the year 1819 an acidic substance greatly resem- 

 bling the tartaric acid discovered by Scheele had been 

 found in the grape-juice vats of Thann in Alsace, and 

 in 1826 it was investigated by Gay Lussac, who was 

 obliged, however, to confess himself puzzled as to the 

 nature of the substance. Gmelin also examined it 

 in 1829, with a similar result, but he at least gave it 

 a name, Traubensaure — acid of grapes — which was 

 translated as racemic acid in France and England. 

 Subsequently Berzelius tackled it, and got so far as to 

 prove that its empirical composition was C 4 H 6 6 , the 

 same as that of Scheele's tartaric acid, this observa- 

 tion being indeed the introduction of the principle 

 of isomerism into chemistry. Shortly afterwards 

 Biot, who subsequently became the close friend and 

 admirer of Pasteur, in the course of his pioneer work 

 on the rotation of the plane of polarisation of light by 

 certain specific substances, examined for rotatory 

 power both this racemic acid and tartaric acid. He 

 found that the latter, both the crystals and their 

 solution in water, rotated the plane of polarisation to 

 the right, but that racemic acid was optically inactive. 



It was at this juncture that Pasteur took up the 

 study of the subject, concentrating his attention first 

 on racemic acid and its salts. One of the most readily 

 obtainable is sodium hydrogen racemate, and when 

 to the solution of this salt in water ammonium hydrate 

 is added, the salt sodium ammonium racemate crystal- 

 lises out on standing, its composition being C 4 H 4 6 

 NaNH 4 . 4.H 2 0. It was in examining these crystals 

 that Pasteur made the initial discovery which led to 

 all the rest. For he observed that in some of the 

 crops obtained all the individual crystals were either 

 right-handed or left-handed, the two varieties being, 

 when analogously equally developed, the mirror- 

 images of each other. Moreover, crystals of either kind 

 at will could be obtained from a metastable (saturated 

 at 28° C. and cooled to ordinary temperature) solution 

 of the salt by touching the solution with a crystal of 

 the desired variety. The crystals belong to the 

 rhombic bisphenoidal class 6, the faces of comple- 

 mentary bisphenoids being present on opposite sides 

 on the two kinds of crystals. 



On collecting crystals of each variety apart, redis- 

 solving them and recrystallising, the fresh crystals 

 proved to be of the same kind as those dissolved ; 

 and on precipitating a solution of each variety with a 

 soluble lead salt and decomposing the precipitated 

 lead salt by means of sulphuretted hydrogen, Scheele's 

 ordinary dextro-tartaric acid was obtained in one 

 case, while the other variety gave quite a new form of 

 tartaric acid, the crystals and solution of which rotated 

 the plane of polarisation to the left. This was, in 

 fact, the isolation by Pasteur of laevo-tartaric acid. 

 Further, on mixing the two separate acids thus derived, 

 right-handed and left-handed, he noticed that heat 

 was evolved, a molecular combination of the two 

 varieties occurring, the product being racemic acid, 

 which crystallised out with a molecule of water of 

 crystallisation, C 4 H 6 6 . H 2 0. Thus he discovered the 

 true nature of racemic acid, namely, that it is a mole- 

 cular compound of the two optically active tartaric 

 acids, the two varieties exactly neutralising each other 

 and producing thereby optical inactivity. 



Pasteur must have had some considerable crystallo- 

 graphic knowledge, for he measured crystals of dextro- 

 tartaric acid and made observations with laevo-tartaric 

 acid which were adequate to prove that its crystals 

 were the mirror-images of those of the ordinary dextro 

 acid. The crystals belong to the sphenoidal class 4 

 of the monoclinic system. Typical crystals of the two 

 varieties are shown in the accompanying Figs. 1 and 

 2, and it will be clear that the dextro variety, Fig. 1, 



exhibits the right clino-prism {on}, while the lsevo 

 variety, Fig. 2, has only the left clino-prism {01 1} 

 developed, of the two possible sphenoids, the dis- 

 tinctive forms of this class of lower than holohedral 

 symmetry. 



These crystals of the two optically active tartaric 

 acids are anhydrous, corresponding to the formula 

 C 4 H 6 G . On the other hand, racemic acid, as already 

 mentioned, crystallises with a molecule of water, and 

 the crystals are quite different and of only one kind, 

 belonging to the pinakoidal holohedral class 2 of the 

 triclinic system, as subsequently established by De 

 In Prevostaye. The combination of the two optically 

 active varieties, however, is so loose that the separation 

 into the two kinds already described is possible, under 

 the specific conditions stated. 



