INTRODUCTION. XV 



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 intro- 

 duction of these spirally-arranged facets. They con- 

 stituted 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 outward crystalline form, one of them, 

 nevertheless, causing the plane of polarisation to 

 rotate, while the other did not, Pasteur, remember- 

 ing, no doubt, the observations just described, insti- 

 tuted a search for facets like those discovered in 

 rock-crystal, and which, without altering chemical 

 constitution, destroyed crystalline identity. He first 

 found such facets in the tartrates, while he subse- 

 quently 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 para- 

 tartrate was dissolved, exactly neutralised the other. 



Prior to Pasteur the left-handed tartrate was 

 unknown. Its discovery, moreover, was supplemented 

 by a series of beautiful researches on the compounds of 

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



