THE VOLATILE PART OF PLANTS. 45 



cellulose dissolves. On adding much water to the solu- 

 tion there separates a white substance which has the same 

 composition as cellulose, but is readily converted into 

 dextrin by cold dilute acid. This form of cellulose as- 

 sumes a fine blue color when put in contact with iodine- 

 tincture and sulphuric acid. 



EXP. 23. Fill a large test-tube first with water to the depth of two or 

 three inches. Then add gradually three times that bulk of oil of vitriol, 

 and mix thoroughly. When well cooled pour a part of the liquid on a 

 slip of unsized paper in a saucer. After some time the paper is seen to 

 swell up and partly dissolve. Now flow it with solution of iodine,* 

 when these dissolved portions will assume a fine and intense blue color. 

 This deportment is characteristic of cellulose, and may be employed 

 for its recognition under the microscope. If the experiment be re- 

 peated, using a larger proportion of acid, and allowing the action to 

 continue for a considerably longer time, the substance producing the 

 blue color is itself destroyed, and addition of iodine has no effect.f Un- 

 altered cellulose gives with iodine a yellow color. 



Paper superficially converted into amyloid constitutes vegetable 

 parchment, which is tough and translucent, much resembling bladder, 

 and very useful for various purposes, among others as a substitute for 

 sausage " skins." 



EXP. 24. Into the remainder of the cold acid of Exp. 23 dip a strip of 

 unsized paper, and let it remain for about 15 seconds ; then remove, and 

 rinse it copiously in water. Lastly, soak some minutes in water, to 

 which a little ammonia is added, and wash again with pure water. 

 These washings are for the purpose of removing the acid. The success 

 of this process for obtaining vegetable parchment depends upon the 

 proper strength of the acid, and the time of immersion. If need be, 

 repeat, varying these conditions slightly, until the result is obtained. 



The denser and more impure forms of cellulose, as they 

 occur in wood and straw, are slowly acted upon by chem- 

 ical agents, and are not easily digestible by most animals ; 

 but the cellulose of young and succulent stems, leaves, 

 and fruits is digestible to a large extent, especially by 

 animals which naturally feed on herbage, and therefore 

 cellulose is ranked among the nutritive ingredients of 

 cattle-food. 



Chemical composition of cellulose. This body is acom- 



* Dissolve a fragment of iodine as large as a wheat kernel in 20 c. c. of 

 alcohol, and add 100 c. c. of water to the solution. 



t According to Grouven, cellulose prepared from rye straw (and im- 

 pure?) requires several hours' action of sulphuric acid before it will 

 strike a blue color with iodine (Zter Salsmunder liericht, p. 467). 



