148 VEGETABLE PRODUCTS. 



will resist ordinary atmospheric agents. This is undoubtedly 

 owing to the constitution of the materials contained in the wood, 

 or to products formed after the tree is dead. The reason that 

 heart-wood will last longer than sap-wood is principally owing 

 to the fact that the nitrogenized materials, which rapidly decay, 

 are all removed from the old wood, and are abundant in the sap 

 of the new. The hardness of wood is very various, owing, in a 

 great measure, to the compactness of the woody layers and the 

 amount of sclerogen deposited in the woody fibers. 



284. By replacing three atoms of the Hydrogen in Cellulose, 

 by three atoms of Nitric acid, we have gun-cotton (Xyloidine, 

 Pyroxiline), C 12 , H 7 3 (NO s ), 8 . This is accomplished by mix- 

 ing two parts of Nitrate of Potash and three parts of Sulphuric 

 acid ; and into the solution put some perfectly clean and loose 

 cotton, and after a few minutes the change is effected. The 

 cotton being thoroughly washed and dried, at a very moderate 

 heat, is gun-cotton. 



It is soluble in Sulphuric Ether, and the solution is called 

 Collodium A thin coat of it forms immediately a cuticular 

 membrane, which is often formed over sores or abraded surfaces 

 when collodion is applied to them. It is also used in Photogra- 

 phy, furnishing, with other materials, a sensitive coating on glass. 

 It explodes violently, leaving no residuum, being resolved into 

 C0 2 , CO, N0 2 , and HO. 



285. Starch, C 12 , H l0 , O 10 . Next to cellulose, starch is un- 

 doubtedly the most abundant and universal product of vegeta- 

 tion. It exists in the cells of plants, sometimes nearly filling 

 them. Fig. 167 exhibits a cell with starch grains in it. 



Fig. 1C8. 



Fig. 167. 



C 



Q^r^ 



The grains are regularly formed, being composed of concen- 

 tric layers with an insoluble external covering, and each grain 

 having one or more points on its surface, resembling the bilum 

 of a seed. (Fig. 168, a, potato grains; Fig. 168, b, wheat 

 grains; Fig. 168, c, rice grains.) It is an interesting employ- 



284. "What is gun-cotton? What is collodion? For what used? — 285. 

 What is the constitution of starch? Where does it exist? Describe the 

 grains. Where found? What are the properties of starch? 



