THE VOLATILE PART OF PLANTS. 59 



EXP. 23. To prepare parchment paper, fill a large cylindrical test tube 

 first to the depth of an inch or so with water, then pour in three times 

 this bulk of oil of vitriol, and mix. When the liquid is perfectly cool, im- 

 merse into it a strip of unsized paper, and let it remain for about 15 sec- 

 onds ; then remove, and rinse it copiously in water. Lastly, soak for 

 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 experiment depends upon the proper 

 strength of the acid, and the time of immersion. If need be, repeat, va- 

 rying these conditions slightly, until the result is obtained. 



Prolonged contact with strong sulphuric acid converts 

 cellulose into dextrin, and finally into sugar, (see p. 75.) 

 Other intermediate products are, however, formed, whose 

 nature is little understood ; but the properties of one of 

 them is employed as a test for cellulose. * 



EXP. 24. Spread a slip of unsized paper upon a china plate, and pour 

 upon it a few drops of the diluted sulphuric acid of Exp. 23. After some 

 time the paper is seen to swell up and partly dissolve. Now flow it with a 

 weak 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 repeated, 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 or converted into sugar, and 

 addition of iodine has no effect, t 



Boiling for some hours with dilute sulphuric acid also 

 transforms cellulose into sugar, and, under certain circum- 

 stances, chlorhydric acid and alkalies have the same 

 effect upon it. 



The denser and more impure forms of cellulose, as they 

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

 cal 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 in the 

 stomachs of animals which naturally feed on herbage, an<3 

 therefore cellulose ranks among the nutritive substances. 



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

 hol, add 100 c. c. of water to the solution, and preserve in a well stoppered bottle. 



t According to Grouvcn, cellulose prepared from rye straw, (and impure ?) 

 requires several hours' .action of sulphuric acid before it will strike a blue color 

 with iodine, (*ter SatznMnder BertcM, p. 407.) 



