172 PRACTICAL LESSONS IN SCIENCE. 



extensively as a lubricating oil. It burns with luminous flame, 

 and is used in the manufacture of candles. 



Petroleum oil is a mixture of hydrocarbons of the marsh gas 

 and olefiant gas series, which makes it valuable for lubricating 

 and lighting purposes. Bitumen, or asphaltvm, is a hydrocar- 

 bon something like turpentine, which seems to be derived from 

 petroleum oil. It is soluble in turpentine, and is used as a water- 

 proof varnish and in making roadways, walks, etc. 



When incisions are made in the bark and wood of the different 

 varieties of pine, spruce and allied trees, a viscous substance 

 flows out called pitch, gum, turpentine or balsam. It consists of 

 resin mixed with the essential oil of the plant. When this sub- 

 stance is distilled with water the oil passes over and the resin 

 remains, which is called rosin. Turpentine obtained from the 

 pine yields from 75 to 90 per cent, of rosin (C 20 H 30 2 ), and from 

 10 to 25 per cent, of essential oil commonly called spirits of tur- 

 pentine, C 10 H 16 . 



Rosin burns with a smoky flame, and is used in the prepara- 

 tion of lampblack and varnish. The resins generally are in- 

 soluble in water, but dissolve in alcohol, naphtha and the oil of 

 turpentine. The solutions thus obtained are called varnishes. 

 When exposed to the air in a thin layer the solvent evaporates, 

 leaving a layer of resin over the substance which is impervious to 

 air and water. Gum copal, shellac, amber and the balsams are 

 resins similar to common rosin. 



The oils of bergamot, cloves, lemons, tolu, valerian and 

 others are similar to the oil of turpentine, having about the 

 same composition. 



Camphor, obtained from the camphor laurel of Japan, is 

 similar to the resins, having the formula C 10 H 16 0. It is a crys- 

 talline substance, melting at 175 F., burning with smoky flame, 

 and passing into vapor at ordinary temperatures. It is almost 

 insoluble in water, but is dissolved by alcohol and ether. The 

 alcoholic solution is known as spirits of camphor. 



Asafoetida, aloes, gamboge, myrrh and others are gum-resins 

 which exude from plants in a milky state, gradually solidifying 



