LESSONS IN CHEMISTRY. 173 



on exposure to the air. They consist of a mixture of gum, resin 

 and essential oil. They are partially soluble in alcohol, and are 

 used in medicine. 



Caoutchouc is prepared from the milky juice of several varieties 

 of tropical plants. It is insoluble in water, but soluble in tur- 

 pentine and naphtha. It is elastic, and when warmed becomes 

 soft and pliable. It is not acted on by alkalies or dilute acids. 

 In time it oxidizes into a resin, losing its strength and water- 

 proof qualities. It is extensively used in the manufacture of 

 waterproof goods, water tubes, springs, elastic fabrics, gas 

 holders, etc. In many cases it is used in the manufacture of 

 waterproof cloth or felt, which is prepared by dissolving the 

 caoutchouc and spreading it over the cloth or felt, which is then 

 passed between rollers, which causes firm union, waterproofing 

 the cloth and strengthening the caoutchouc. 



Caoutchouc placed in fused sulphur will absorb large quantities 

 of that element, and if heated to about 300 F. it becomes vul- 

 canized, and if heated to a higher degree it at length becomes 

 vulcanite, which is used in the manufacture of combs, etc. Vul- 

 canized caoutchouc is not soluble in turpentine or naphtha, and 

 is not sticky when warmed. 



Gutta percha is similar to caoutchouc in many respects, but is 

 only slightly elastic at ordinary temperatures. It is insoluble in 

 water and a poor conductor of electricity, so that it is much 

 used for water pipes and as an insulator for electric wires, etc. 

 Gutta percha is a hydrocarbon, C 20 H 32 , often mixed with resins. 

 Caoutchouc, C 5 H 8 , has the same composition with the same pro- 

 portion of elements, but of different-sized molecules. Such bodies 

 are said to be isomeric (equal parts) . There are many instances 

 of isomerism among organic compounds. 



Gums, as gum arabic, gum tragacanth appear like resins, but 

 unlike them are soluble in water, and contain oxygen, gum 

 arabic having the formula C 12 H 22 O n , and tragacanth C 15i H 20 10 , 

 differing by only one molecule of water. 



Wood, free from sap, consists of cellulose (C 6 H 10 5 ),lignine and 

 some mineral substances. The cellulose composes the wood cells, 



