INTKODUCTION. 17 



weight, and show no essentially different chemical pro- 

 perties, and yet conduct themselves somewhat differ- 

 ently in connection with certain physical properties, 

 especially in the action on polarized light, are said to 

 be physically isomeric. 



By the expression homologous bodies, are understood 

 such bodies as conduct themselves in their chemical 

 properties in a similar manner, and differ in their com- 

 position by the group CH 2 , or a multiple of it. We 

 are, for instance, acquainted with a series of compounds 

 which, in their conduct, show the greatest similarity to 

 ordinary alcohol, and of which each succeeding member 

 differs in its composition from the preceding by the group 

 CH 2 , as may be seen in the following schedule : 



CH 4 .... Wood-spirits, methyl alcohol, 



C 2 H 6 .... Spirits of wine, ethyl alcohol, 



C 3 H 8 .... Propyl alcohol, 



C 4 H 10 .... Butyl alcohol, 



C 5 H 12 .... Fusel-oil, amyl alcohol, etc. 



Another series, of which acetic acid is the principal 

 representative, runs parallel to this : 



CH 2 2 Formic acid, 



C 2 H 4 2 Acetic acid, 



C 3 H 6 2 Propionic acid, 



C 4 H 8 2 Butyric acid, 



C 5 H 10 2 Valeric acid, etc. 



For several of these homologous series, as, for instance, 

 for the two mentioned, experience has shown that the 

 following law exists : The boiling point of a compound 

 is 19-20 higher if it contains CH 2 more than another 

 member of the series. The boiling point of ethyl 

 alcohol is, for instance, 78; that of normal amyl 

 alcohol, which differs from ethyl alcohol by 3 x CH 2 , 

 should, according to this law, be 3 x 19 higher. The 

 observed boiling point is 137. 



In connection with other homologous series, a simi- 

 lar conformity is observed, but the difference in the 

 boiling points effected by the addition of every CH 2 is 

 not the same. With the hydrocarbons, which are 



2* 



