THE ATOMIC VIEW OF NATURE. 



431 



growing volume of chemical knowledge ; that the concep- 

 tion of the atom must be extended and more closely de- 

 fined ; that the proportions of weight were inadequate for 

 the purpose of distinguishing and identifying the many 

 organic compounds ; and especially that the relations of 

 volume and the arrangements of particles of matter in space 

 must be taken notice of, if the atomic view of matter was 

 to be made further serviceable for scientific purposes. That 

 purely geometrical relations, such as can be grasped only 

 by our space conceptions, are of importance in the chem- 

 ical composition of substances, was very evident, for 

 instance, in some of the optical properties of crystallised 

 organic substances. The discoveries of Pasteur, published 

 in 1850, mark in this respect an epoch in science. 1 He 

 showed that there exist chemical substances which are 

 different, but only as a right-hand glove differs from a 

 left-hand one, a right-handed screw from a left-handed, 



81. 



Develop- 

 ment of the 

 atomic view. 



32. 



Pasteur's 



1 A special line of " physical " or 

 "mechanical " reasoning which bears 

 upon the atomic view of matter be- 

 gan with Biot's discovery in 1815 

 that certain fluids notably organic 

 have the property of rotating the 

 plane of polarisation of light which 

 passes through them. Later on he 

 extended this observation to the 

 vapours formed by such fluids. 

 Faraday found in 1846 that sub- 

 stances which are optically "in- 

 active " become active in the 

 manner described under the influ- 

 ence of powerful electro-magnets. 

 An explanation of the phenomenon 

 by Fresnel, which was based upon 

 crystalline structure, would for 

 liquids and vapours have to be 

 applied to the structure of the mole- 

 cule itself. Pasteur found in 1850 

 that there exist two modifications 



of tartaric acid, which differ in this 

 only, that one of them turns the 

 plane of polarisation to the right, 

 the other to the left, and that a 

 mixture of both in the proper pro- 

 portions is inactive. As far back 

 as 1860, in his ' Lecons de Chimie,' 

 he put the question, "whether 

 the atoms in tartaric acid are ar- 

 ranged like the turns of a right- 

 handed screw, or situated in the 

 corners of an irregular tetrahedron, 

 or have they any other asymmetrical 

 grouping ? . . . There can be no doubt 

 that the atoms have an unsym- 

 metrical arrangement after the fash- 

 ion of mirrored images which cannot 

 be made to fall into each other " 

 (quoted by Van't Hoff, ' Die Lager- 

 ung der Atoine im Raume,' Ger- 

 man translation, 2d. ed., p. 9). 



