452 CONSIDERATION OF CARBON COMPOUNDS. 



in other words, when one substance contains some multiple of 

 the number of each of the atoms contained in the molecule of the 



other. 



For instance, some volatile oils have the composition C 20 H 32 , which 

 is double the number of atoms contained in oil of turpentine, C 10 H 16 ; 

 acetylene, C 2 H 2 , is polymeric with benzene, C 6 H 6 , and styrene, C 8 H 8 ; 

 formaldehyde, CH 2 O, acetic acid, C 2 H 4 O 2 > lactic acid, C 3 H 6 O 3 > and 

 glucose, C 6 H 12 O 6 , are polymeric compounds. 



Stereo-isomerism. There has long been known a number of 

 bodies having the same molecular and constitutional formulas (i. e., 

 behaving alike chemically), but which exhibit differences in prop- 

 erties, as, -for instance, in their behavior toward polarized light 

 and in the form of their crystals. The explanation at present 

 given of these differences is based on this assumption : that the dif- 

 ferent atoms or radicals in combination with a carbon atom may 

 occupy toward it different relative positions, and that actually 

 they do. 



In order to understand what is meant by this statement we should 

 bear in mind that we represent the grouping of our atoms on the 

 flat surface of paper, while actually the formation of molecules takes 

 place in space i. e. y in three directions. If we assume, for instance, 

 that four different radicals are in combination with a carbon atom, 

 we can well imagine that the relative positions in which these radicals 

 are grouped around the carbon atom have an influence on the nature 

 of the compound. There are bodies which contain the same elements 

 in the same quantities but in which the molecular structures seem to 

 be reversed, precisely as they would be if seen directly and then 

 observed after reflection from a mirror. In fact, there are known 

 isomeric bodies the crystals of which seem to exhibit exactly that 

 relation to each other. 



The term stereo-isomerism is, therefore, used for that kind of 

 isomerism found in substances which contain apparently the same 

 radicals, show practically the same chemical behavior toward other 

 agents, but differ in certain physical properties. Of stereo-isomeric 

 substances may be mentioned 2 malic acids, 3 lactic acids, 4 tartaric 

 acids, etc. (For details of stereo-isomerism the student is referred 

 to works treating more fully on this subject.) 



Various modes of decomposition. The principal changes which 

 a molecule may suffer are as follows : 



