REACTIONS OF CELLULOSE. 33 



color, however, is given even by fresh solutions of iodine to 

 cellulose which has been previously treated with certain chemi- 

 cal agents, for instance, strong sulphuric acid. A convenient 

 method of employing this reaction as a test for cellulose is to 

 thoroughly moisten the object with a dilute solution of iodine, 

 and then to apply strong sulphuric acid, upon which the cellulose 

 immediate!}' turns bright blue. It is sometimes advantageous 

 to dilute the sulphuric acid employed, either with water or with 

 glycerin ; but for most cases the concentrated acid is the best. 



Schulze's solution of iodine, better known as chloroiodide of 

 zinc, used alone, gives with pure cellulose a blue color inclining 

 to purple. This reaction, though not always so prompt as the 

 other, is generally more manageable, and, on the whole, more 

 satisfactory. 



In a few instances the cell-membrane becomes yellowish- 

 brown throughout, upon the application of an iodine solution, 

 a reaction which might be easily mistaken for that which albu- 

 minoids give ; that the color, however, is not here due to their 

 presence, appears on subjecting the tissue to the action of 

 Millon's reagent. Vertical sections of the stem of Begonia, as 

 noticed by Nageli, afford an instructive example of this. 1 



1 That the yellow color imparted by iodine has been otherwise interpret, 

 will ii|i|M-ar from the following : 



Harting(Ann. des Sc. nat., ser. 3, tome v. p. 328) states, that "all lignifitJ 

 cells have Protein matters in their walls." 



Mohl (The Vegetable Cell, p. 25) says: "Nitrogenous compounds <Io nci 

 occur in the membranes of cells which are just at the commencement of their 

 development, for these are not coloured yellow by tincture of iodine, yet hardly 

 a full-grown cell is met with in which this is not the case." 



It is held by Nageli that vegetable cell-membranes consist, in some in- 

 stances, of two isomeric substances, unequally soluble, which are intimately 

 commingled. One of these is soluble in cold water, more easily in hot water, 

 and sometimes needs for its complete extraction a dilute acid. From the solu- 

 tion iodine throws down a blue or bluish-green precipitate. 



A synoptical table, based on differences in solubility of cellulose and its 

 modifications, and in their behavior towards iodine, has been constructed by 

 Nageli. The part of the table which is given below affords excellent practice 

 for the beginner. 



I. DIFFERENCES IN SOLUBILITY. 



(1) In cold water, becoming swollen ; in hot water, disappearing, vtqctaWe 

 mucilage ; c. </., in the outer layer of the cells forming the testa of quince seeds 

 and tJwse of flax, 



(2) Soluble in concentrated sulphuric acid, and in cuprammonia; e. g. t cotton- 

 hairs, bast-fibres, etc. 



3 



