16 . INTRODUCTION. 



ing to the above method of consideration, should have 

 eight free affinities ; in most reactions, however, it 

 conducts itself as a saturated compound. 



It is frequently the case that two organic bodies 

 contain the same elements in the same proportion by 

 weight, and still have entirely different physical and 

 chemical properties. In general such bodies are called 

 isomeric. For this relation there may be two different 

 causes, viz. : 



1. A dissimilarity of constitution, i. e., a dissimi- 

 larity in the method of 'grouping or joining of the 

 atoms in the two bodies, as, for instance, in ethyl for- 

 mate, methyl acetate, and in propionic acid. All three 

 of these compounds have the formula C 3 H 6 2 . In ethyl 

 formate, however, the atoms are grouped together 

 according to the formula CHO.O.C 2 !! 5 , or further 

 reduced CHO.O. CIP.CH 3 , whereas in methyl ace- 

 tate the method of grouping is C 2 H 3 O.O.CII 3 or 

 CHACO.O.CH 3 , and in propionic acid C 3 H 5 .O.OH or 

 CH 3 .CH 2 .CO.OH. Ammonium cyanate CIsr.O.KH 4 and 



urea CO-< || bear a similar relation to each 



( ISTH 2 



other. In the latter case the change in the arrange- 

 ment of the atoms from the first manner of grouping 

 to the second takes place spontaneously at the ordinary 

 temperature. 



Such bodies are called metameric or isomeric in the 

 narrower application of the word. 



Or, 2. A different molecular weight, as, for instance, 

 acetic aldehyde and butyric acid, which both contain 

 the same percentages of their constituents. The mole- 

 cular weight of aldehyde C 2 H 4 is, however, only half 

 as large as that of butyric acid C 4 H 8 2 . According 

 to the same principle acetic acid C 2 H 4 2 , and grape- 

 sugar C 6 H 12 6 , acetylene C 2 H 2 , and benzol C 6 H 6 , and 

 many other compounds, are isomeric. Such bodies are 

 called -polymeric. 



Compounds which contain the same elements in the 

 same proportions by weight, have the same molecular 



