374 CHEMISTRY. 



525. Composition. The volatile oils are divided into 

 three classes, according to their composition: 1. Oxygen- 

 ated oils. These, which are by far the most numerous class, 

 are composed of carbon, hydrogen, and oxygen. 2. Non- 

 oxygenated oils, which are composed only of carbon and 

 hydrogen, and are therefore called hydrocarbons. The 

 principal of these are the oils of turpentine, savin, juniper, 

 lemons, etc. 3. Sulphuretted oils, which are composed of 

 carbon, hydrogen, and sulphur. Nitrogen is a component 

 of some of them. These oils exist in mustard, horseradish, 

 garlic, onions, hops, etc. They have a very pungent smell, 

 causing lachrymation, and are so acrid that they raise blis- 

 ters when applied to the skin. 



526. Camphor. Camphor is obtained by distilling with 

 water the wood of the laurus camphora. This forms when 

 pure a white, crystalline, translucent solid, having a pecul- 

 iar odor which is familiar to every one. It gradually sub- 

 limes at ordinary temperatures, and often forms beautiful 

 crystals on the sides of the bottles in which it is kept. 



527. Resins. Where an essential or volatile oil is ex- 

 posed to the air, a part of it evaporates, diffusing an odor, 

 but a part combines with the oxygen of the air, forming a 

 resin. The pure rosin, or colophony, is thus produced from 

 the oil of turpentine. It is really a mixture of two acids. 

 This oxidation is, however, only partial, so that the turpen- 

 tine when gathered is a mixture of the oil and the resin. 

 Some of the resins are called balsams. The resins are very 

 indestructible, and have also the power of preserving other 

 substances from decay. The mummies found in the pyra- 

 mids of Egypt are bodies which were embalmed with res- 

 ins. Amber furnishes the most striking illustration of this 

 indestructibility. This resin was formed in the early ages 

 of the world, it having survived the destruction of the trees 

 from which it exuded. Insects are often seen inclosed in 



