October 12, 1900.] 



SCIENCE. 



541 



ism of tartaric and racemic acids was rec- 

 ognized by Gay-Lussac in 1826 and by 

 Berzelius in 1830, but notliing in their ob- 

 servations indicated that this isomerism 

 was in any way peculiar and of a finer 

 kind than that existing in other cases. 

 Still earlier, in 1815, Biot had found that 

 tartaric acid and various other organic 

 bodies, such as sugar, camphor, turpentine, 

 possess the power of rotating the plane of 

 polarization of light, and as this property 

 is shown by them in the dissolved state, it 

 was clear that it must be due, not to crys- 

 talline structure, but to intramolecular con- 

 stitution. In 1841, de La Provostaye in- 

 vestigated the crystallography of tartaric 

 and racemic acids and their salts without 

 noticing any difference between the two 

 series, while in 1844 Mitscherlich examined 

 the double sodium ammonium salts of these 

 acids crystallographically with the same 

 result. Referring to the discovery of Biot 

 that the tartrates are dextrorotary, the 

 racemates indiiferent, Mitscherlich says : 

 " ISTevertheless, the nature and number of 

 the atoms, their arrangement and their dis- 

 tances apart are the same in both bodies." 

 In 1848, Louis Pasteur, who was then a 

 chemical student just beginning independ- 

 ent work, turned his attention to the study 

 of crystals as offering a possible assistance 

 to him in his chemical researches. With 

 no expectation of making a discovery, with 

 a view to practice solely, he began by re- 

 peating de La Provostaye's work of seven 

 years before, as far as it related to tartaric 

 acid. He soon observed a fact which had 

 escaped the former, this being that crystals 

 of the tartrates possess certain hemihedral 

 faces, and further that the hemihedrism is 

 in the same sense in all the tartrates. 

 Led by the observation of Hauy and Weiss, 

 on the existence of right- and left-handed 

 hemihedrism in quartz, of Biot, on the ex- 

 istence of dextro- and Isevo-rotary quartz, 

 and of Sir John Herschel, that the crystal- 



lographic difference of the two kinds of 

 quartz is associated with a corresponding 

 difference in the sense of their optical rota- 

 tion, he undertook an investigation designed 

 to ascertain whether in the various crystal- 

 line organic bodies possessing optical rota- 

 tion in solution, this property is always ac- 

 companied by hemihedrism, and whether 

 absence of the one implies corresponding 

 absence of the other. He examined the 

 crystals of the optically indifferent racemic 

 acid and its salts ; none of these showed 

 hemihedrism. Mitscherlich had failed to 

 observe the hemihedrism of the active so- 

 dium ammonium tartrate and found that 

 its crystals differ in no wise from those of the 

 corresponding inactive racemate. Herein 

 was an apparent exception to the rule, and 

 Pasteur, therefore, repeated Mitscherlich's 

 work. He found the hemihedrism of the 

 sodium ammonium tartrate which had es- 

 caped the eye of Mitscherlich, but he also 

 found — and this is the observation which 

 entitles him to be regarded as one of the 

 founders of chemistry in space — that ex- 

 ceptionally the double racemate also showed 

 hemihedral faces, but that while half of the 

 crystals were hemihedral in a right-handed 

 sense, the other half were so in the oppo- 

 site sense.* Carefully separating the two 

 kinds, dissolving them and placing the so- 

 lutions in the polarimeter, he found, to his 

 great surprise and delight, that the one so- 

 lution was dextro-, the other Isevo-rotatory. 

 From the latter he prepared Isevo-tartaric 

 acid, the hitherto unknown isomer of com- 

 mon tartaric acid. Mixing the two acids in 

 equal quantities, he regenerated the inac- 

 tive racemic acid. 



* The inversion temperature of sodium ammonium 

 racemate is 27° C. (van't HoH and van Deventer, 

 Zeit. physik. Cliem. 1, 173). Above this temperature 

 the racemate is stable, below it, the mixed dextro- and 

 Isevo-sodium ammonium tartrates. Mitscherlich's fail- 

 ure to detect the facts afterwards observed by Pasteur 

 may therefore have been due, not to erroneous obser- 

 vation, but to improperly selected temperatures. 



