302 CHEMISTEY. 



relations of which are not yet well enough understood to 

 bring them within a scientific system: such are gelatin, 

 albumen, vegetable resins, and other compounds formed in 

 the bodies of plants and animals. . 



We will now review briefly the chemical relations of the 

 above-named groups, reserving details until we meet with 

 them again farther on. 



419. Hydrocarbons. You have already become somewhat 

 familiar with two important hydrocarbons in the first part 

 of this work marsh gas, CH 4 , and defiant gas, C 2 H 4 . 

 But besides these there is an immense number of other 

 bodies, solid and liquid as well as gaseous, made up solely 

 of C and H in various proportions. No two elements are 

 capable of combining in so many different forms as carbon 

 and hydrogen. On page 324 you will find a table giving 

 the names and formula of a large number of hydrocarbons 

 of the so-called Marsh Gas Series, occurring in American 

 petroleum. On examining the formula you will notice 

 that in each of the two series the hydrocarbons differ by 

 exactly CH 2 ; that is, each successive formula may be ob- 

 tained by adding CH 2 to the preceding one ; this is another 

 and striking example of isomerism. 



Besides the long series of hydrocarbons given on page 

 324, there are several other series differing from each other 

 by H 2 , and the members in each differing by CH 2 . Thus 

 olefiant gas, C 2 H 4 , belongs to a series which takes its name 

 from this its important member. In the following table 

 you have this series with the formula?, and the correspond- 

 ing alcohol and acid, to which we will have occasion to re- 

 fer a little later. In this table olefiant gas is called Ethy- 

 lene. 



