A LABORIOUS YOUTH 33 



it and which revolutionised all the hitherto ac- 

 quired knowledge of molecular composition. 

 Pasteur resolved to find out why the para- 

 tartrates did not deflect light; he analysed 

 them anew, at great length, and he perceived 

 that the double paratartrates of sodium and 

 ammonia, like those of sodium and potassium, 

 had hemihedric crystals, but that the ones were 

 left-handed and the others right-handed. This 

 seemed to contradict his first discovery, and it 

 was at this point in his labours that his de- 

 cisive experiment took place. "In spite of 

 much that was unexpected in this result," he 

 said, "I none the less continued to follow up 

 my idea. I carefully separated out the right- 

 hand hemihedric crystals and the left-hand 

 hemihedric crystals, and I observed separately 

 the effect of their solutions in the polarising 

 apparatus. I then saw, with no less surprise 

 than delight, that the right-hand hemihedral 

 crystals deflected the plane of polarisation to 

 the right, and the left-hand hemihedral crys- 

 tals deflected to the left; and, when I took an 



