H. VON MOHL ON CELLULOSE. 101 



required a long continuance of the boiling before the iodine would 

 produce the blue colour. 



The organs above mentioned, forming the surface of plants, 

 especially the cuticular layer of the epidermis and the cork of 

 the cork oak, and in a less degree the corks of the other plants 

 enumerated, stand, in respect to the chemical properties of the 

 substances combined with their cell-membranes, preventing the 

 reaction of cellulose, in opposition to all those elementary organs 

 which form the internal tissues of plants. In these also the re- 

 action of cellulose is very frequently partially or entirely pre- 

 vented by compounds combined with them, but caustic potash is 

 not the proper means of reducing the cellulose of these organs 

 into a condition capable of the reaction. Even when, as in many 

 cases, for example in the secondary layers of many wood-cells, 

 such as those of the wood of Buxus, potash has this effect, the 

 result is often waited for in vain, and in case of success, the mem- 

 branes boiled with caustic potash but seldom acquire a pure blue 

 colour with iodine, and a yellow or brown colour is mostly min- 

 gled with it. On the other hand, the application of nitric acid 

 is always attended with complete success. 



The effect of this acid is perhaps most perfect when the plant 

 to be investigated is allowed to macerate for a long time in 

 dilute acid, at ordinary temperatures ; but since in solid woods, 

 when even small fragments are placed in the acid, several months 

 or even a year may easily elapse before the effect of the acid has 

 completely developed itself, this method is scarcely applicable 

 when a large series of observations is to be made with this agent. 

 I therefore substitute a boiling of the substance to be examined 

 in moderately strong acid, for the long-continued maceration ; 

 by this means the desired effect is very rapidly obtained, only in 

 many plants the risk is run of dissolving the cell-membrane, or 

 at least some of its layers, by continuing the boiling too long. 

 This inconvenience, however, may be avoided by a little care ; 

 for in general the colour of the vegetable structure affords a mark 

 by which we can detect whether the required effect of the acid 

 has commenced and the boiling is to be stopped, or it is to be 

 continued for a longer time. At first, the acid ordinarily pro- 

 duces in the fragment of vegetable substances placed in it, 

 immediately it is heated, a yellow or brown colour, accompanied 

 by considerable effervescence and frequently with the formation 



