88 AN INTRODUCTION TO SCIENCE 



was known at that time that certain compounds when in 

 solution rotate the plane of polarized light. The undula- 

 tions which constitute a ray of light are in all planes ; but if 

 the ray is passed through a plate of the semi-transparent 

 mineral tourmaline, cut parallel to its axis, only those vibra- 

 tions which are in planes coinciding more or less with the 

 axis of the crystal pass through it to vibrations in other 

 planes tourmaline is opaque and, in passing through, 

 they are turned until they are all quite parallel. The mineral 

 acts as an optical sieve. If now these polarized rays are 

 passed through certain substances in solution, or in the 

 CKystalline form, they are twisted to the right or to the left. 

 Pasteur discovered that there are two kinds of tartaric acid, 

 distinguished when in solution by the fact that the one 

 rotates a ray of polarized light to the right and the other 

 rotates it to the left ; and he found that these two forms of 

 the acid (which appear to be absolutely identical in chemi- 

 cal properties as well as in specific gravity and other 

 physical properties) differ when crystallized to this extent 

 the laevo-rotary crystals look like the dextro-rotary when 

 they are seen in a looking-glass. They are reversed or 

 enantiomorphic, in the language of crystallography. This, 

 of course, implies that they are asymmetrical. 



There are two forms of ethylidene lactic acid, to take 

 another example, the one dextro-rotary and the other Isevo- 

 rotary. In this substance the tetravalent atom, carbon, 

 has its four affinities satisfied with H, OH, CH 3 , and COOH, 

 respectively. To gain an idea of the way in which these 

 radicles are attached to the carbon atom of what is meant 

 by stereochemistry the reader may cut out a tetrahedron 

 in wood. If he then sticks pins with little heads (H = i) 

 and with big heads (C = 12 and O = 16) into the corners of 

 the four-sided block to represent the four radicles with 

 which the carbon is united, the lob-sided model which he 

 makes may stand for one of the two varieties of lactic acid, 

 say the dextro-rotary. If the dextro-rotary model be held 



