ULTIMATE CONSTITUTION OF MATTER 87 



same terms in which a naturalist describes the struggle for 

 existence. 



Each year our conceptions with regard to the structural 

 constitution of matter, its architecture, become more defi- 

 nite. With the mind's eye we not only see it composed of 

 separate molecules, but we can tell approximately how the 

 atoms are placed in the molecule. We can figure to our- 

 selves the shape of the atom-groups. 



For a long time chemists have denoted compounds by 

 graphic formulae. They have replaced the name of a sub- 

 stance by an ideo-graph, which shows the number of atoms 

 of each element in its molecule. Thus, ammonia is NH 3 ; 

 and aniline is written N (C H 5 ) H 2 to show that one of 

 the three H's in ammonia is replaced by the radicle, ben- 

 zene. Various diagrams are made to indicate that some 

 elements can unite with one combining unit of hydrogen, 

 while others will take two, three, or four. The atoms are 

 represented as having one or more affinities, to which other 

 atoms or radicles can be attached. The atom of carbon is 

 tetravalent, and since each of its four affinities may be satis- 

 fied with a different element or group of elements, its 

 compounds are exceedingly complex. The molecules of 

 all so-called "organic" bodies are clusters of carbon-atoms 

 united with hydrogen, oxygen or nitrogen, or with all three 

 of these elements, and in rare cases with a metal in addi- 

 tion. Now, it is clear that a molecule must occupy three 

 dimensions in space ; and if the chemist wishes to picture 

 its form he must use, not a flat diagram, but a solid model. 

 He must take a stereoscopic view of the molecule. Hence 

 the science of the architecture of matter of the position of 

 atoms in space, is termed stereochemistry. 



That stereochemistry is more than an arbitrary system of 

 symbols, that it is really possible to ascertain the relative 

 positions of the atoms which compose a molecule, and 

 therefore to form a conception of its shape, was first indi- 

 cated by an observation made by Pasteur fifty years ago. It 



