102 H. VON MOHL ON CELLULOSE. 



of vapours of nitrous acid, which colour however soon gives 

 place to a pale yellow, or a complete bleaching of the preparation. 

 When this bleaching takes place, the desired effect is generally 

 attained. I then brought the preparation, if it had not already 

 been boiled between two slips of glass*, on to a glass slider, 

 washed it with water, either dried it perfectly by a moderate 

 heat, or saturated the acid with ammonia, wetted the dried pre- 

 paration with strong tincture of iodine, let it dry up in the air, 

 and wetted it once more with water for microscopic examination 

 and to produce the blue colour. Sometimes it was necessary to 

 repeat the wetting with tincture of iodine, or to moisten the pre- 

 paration, saturated with iodine, with water, and let it dry again 

 several times. The whole process is somewhat tedious, but time 

 is saved, by setting to work with a number of preparations at 

 once, and, when they are saturated with tincture of iodine, letting 

 them dry at leisure, by which means we obtain sufficient material 

 for the whole day's work at the microscopic investigation. I 

 have frequently, to save time, assisted the drying of the prepara- 

 tion saturated with iodine by artificial heat ; as a rule, however, 

 it is more advantageous to allow the evaporation of the iodine to 

 go on at the ordinary temperature of the room, since even a 

 slight heating may easily cause too strong an evaporation of 

 the iodine. 



It is well known that the parenchymatous cells of succulent 

 and young organs, in which the membranes are imbued with a 

 comparatively small quantity of the compounds coloured yellow 

 by iodine, require no preparation to render them capable of as- 

 suming a blue colour with iodine (see my Vermischte Schriften, 

 p. 344). It is different with the parenchymatous cells of older 

 structures which have become saturated with encrusting sub- 

 stances, for instance, the cells of pith and of medullary rays, &c. 

 Very frequently these cannot be coloured blue at all, or only 

 very imperfectly with iodine alone, and do not assume a pure 

 blue tint even with iodine and sulphuric acid, but are tinged with 



* In all cases when it is desired to boil a thin cross section of a vegetable 

 structure in nitric acid, it is advisable to place it in a few drops of acid upon a 

 slip of glass, to lay a thickish covering-glass over it, and then place the glass 

 slip (to avoid cracking it) upon a metal plate which can be heated until the acid 

 boils. The most delicate preparations may be boiled in this manner, while they 

 almost inevitably tear up into fragments when it is attempted to boil them in a 

 glass tube or a platinum spoon. 



