HYDROCARBONS. 



299 



fractional distillation. As the boiling-points of the various compounds 

 differ more or less, they may be separated by carefully distilling off 

 the compounds of lower boiling-points, while noting the temperature 

 of the vapors above the boiling liquid by means of an inserted ther- 

 mometer, and changing the receiver every time an increase of the 

 boiling-point is noticed. This separation of volatile liquid, known 

 as fractional distillation, is, however, not absolutely complete, because 

 traces of substances having a higher boiling-point are simultaneously 

 volatilized with the distilling substance. 



FIG. 39. 



Flasks arranged for fractional distillation. 



For fractional distillation of small quantities of liquids as well as 

 for the determination of boiling-points, flasks arranged like those 

 shown in Fig. 39 may be used. 



Properties of hydrocarbons. There are no other two elements 

 which combine together in so many proportions as carbon and hydro- 

 gen. Several hundred hydrocarbons are known, many of which 

 form either homologous series or are metameric or polymeric. 



Hydrocarbons occur either as gases, liquids, or solids. If the mole- 

 cule contains not over 4 atoms of carbon, the compound is generally 



