H. VON MOHL ON CELLULOSE. Ill 



gr^at deal of water, the dissolved cellulose (which according to 

 this experiment is dissolved as such, and not as dextrine) is pre- 

 cipitated with a beautiful blue colour. But although these means 

 suffice to demonstrate the presence of cellulose, the application 

 of sulphuric acid is not adapted for the settlement of the question, 

 whether in such solid woods the cellulose forms always the 

 principal mass of the membranes and is only saturated with a 

 foreign substance, or the latter is predominant and the cellulose 

 only a very subordinate constituent. In this case the application 

 of nitric acid removes every doubt, inasmuch as the secondary 

 membranes of all wood-cells become bright blue through their 

 entire thickness with iodine, when they have previously been 

 macerated for a long time, or boiled until disorganized, in nitric 

 acid. The compound, therefore, which Mulder termed the " in- 

 termediate wood-substance,^^ never itself forms the intermediate 

 layers of the wood-cells, but is a material infiltrated into them. 

 Since this result is quite universal, I regard it as superfluous to 

 mention particular examples, and confine myself to touching on 

 a few points which may be doubtfuL 



One of these refers to the character of the internal membrane 

 which lines the wood-cells in Taxus and Torreya, and of which 

 the spiral fibres running in these cells form part. This inner 

 coat, as was first shown by Prof. Hartig of Brunswick, resists 

 the action of sulphuric acid very strongly and under its influence 

 is coloured yellow by iodine, whence, relying upon these reagents, 

 as Hartig did, one might be inclined to assume that this membrane 

 was composed of a substance quite different from the interme- 

 diate layer, and contained no cellulose. The latter is by no means 

 the case, for the above-described treatment with nitric acid de- 

 monstrates, by the blue colour which this membrane and its fibres 

 assume, that these also are composed of cellulose. - 



The second point to be adverted to here, relates to the pits, 

 of which it might be doubtful, from the descriptions w^hich 

 Hartig, Harting and Mulder have given of the structure of cells 

 and the characteristics of the " outer wood-membrane," whether 

 the membrane which closes the outer ends of the canals of the 

 pits is always composed of cellulose. In regard to this, there 

 can be no doubt in preparations of the Coniferae which have been 

 treated with nitric acid, since it is found that the pits are closed 



