14 INTRODUCTION. 



The proportion of the molecular weight to the specific 

 gravity of the vapor (molecular weight divided by the 

 specific gravity) is, therefore, for all compounds the 

 same, and is represented hy the constant number 28.9. 



This conformity yields an important and frequently 

 the only means of determining the molecular weight 

 of an organic compound. This is obtained by multi 

 plying the specific gravity found by the number 28.9. 



Carbon is the characterizing element of all organic 

 compounds. In most of these compounds it is in com 

 bination with hydrogen and oxygen, in very many 

 together with nitrogen, sulphur, etc. Furthermore, 

 nearly all other elements can be made constituents of 

 carbon compounds. 



The fact that so great a number and variety of carbon 

 compounds exist is principally due to the tendency, 

 possessed by the atoms of carbon more than by the 

 atoms of any other element, to unite with each other 

 in chains. 



Carbon is tetravalent in all its compounds. &quot;When 

 two or more carbon atoms unite with each other, a 

 portion of the affinities of each atom is used in hold 

 ing the atoms together, so that two atoms of carbon 

 have always less than eight free affinities, three always 

 less than twelve. In most cases the union of several 

 atoms of carbon takes place in such a manner that 

 each of them loses one of its four affinities. Hence, 

 two atoms of carbon have six, three atoms of carbon 

 eight, or in general terms x carbon atoms have 2#-f 2 

 free affinities. 



The valence of any group of atoms containing carbon 

 may be found by subtracting the sum of the affinities 

 of the other atoms present from the affinities of the 

 carbon atoms. The group methyl CIP must be mono- 

 valent, inasmuch as three of the four affinities of the 

 carbon atom are saturated by the affinities of the three 

 monovalent hydrogen atoms. Carbonic oxide CO 

 must be bivalent, because but two of the four affini 

 ties of the carbon atom are saturated by the bivalent 

 oxygen atom. A similar reflection shows us that ethy- 

 lene C 2 H 4 must be bivalent, that acetylene C 2 IP must 



