HIS LIFE AND LABOURS. 177 



moreover, been proved by Biot that there existed two 

 species of rock-crystal, one of which turned the plane 

 of polarisation to the right, and the other to the left. 

 They were called respectively, right-handed and left- 

 handed crystals. No external difference of crystalline 

 form was at first noticed which could furnish a clue to 

 this difference of action. But closer scrutiny revealed 

 upon the crystals minute facets, which, in the one class, 

 were ranged along a right-handed, and, in the other, 

 along a left-handed spiral. The symmetry of the 

 hexagonal prism, and of the two terminal pyramids of 

 the crystal, was disturbed by the introduction of these 

 spirally-arranged facets. They constituted the outward 

 and visible sign of that inward and invisible molecular 

 structure which produced the observed action, and 

 difference of action, on polarised light. 



When, therefore, the celebrated Mitscherlich brought 

 forward his tartrates and paratartrates of ammonia and 

 soda, and affirmed them to possess the same atoms, the 

 same internal arrangement of atoms, and the same out- 

 ward crystalline form, one of them, nevertheless, causing 

 the plane of polarisation to rotate, while the other did 

 not, Pasteur, remembering no doubt the observations 

 just described, instituted a search for facets like those 

 discovered in rock-crystal, which, without altering 

 chemical constitution, destroyed crystalline identity. 

 He first found such facets in the tartrates, while he 

 subsequently proved the neutrality of the paratartrate 

 to be due to the equal admixture of right-handed and 

 left-handed crystals, one of which, when the paratartrate 

 was dissolved, exactly neutralised the other. 



Prior to Pasteur the left-handed tartrate was un- 

 known. Its discovery, moreover, was supplemented by 

 a series of beautiful researches on the compounds of 

 right-handed and left-handed tartaric acid ; he having 



