ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
535 
For these reasons, therefore, viz. the rapidity, certainty, and dis- 
tinctness of its action ; the ease with which it can be made ; its perma- 
nence when made ; and lastly, the difference in its behaviour towards 
tannin and towards gallic acid — for these reasons I am bold enough to 
anticipate the time when, to adapt a hackneyed expression, Nessler’s 
fluid will be regarded as a reagent which no botanical laboratory should 
be without. 
Staining and Imbedding very Minute Objects.* — The preparation 
of microscopically small objects is usually a very unsatisfactory proce- 
dure, but very good results may be obtained, says Dr. E. Overton, by 
adopting the following method : — Suppose the material is a hanging- 
drop cultivation on a cover-glass, as for example unicellular algae, 
Flagellata, pollen-tube, or the like. When the cultivation has reached 
the desired stage of development, the cover-glass is removed and iodine 
vapour allowed to stream over it. Iodine vapour is easily obtained by 
putting some crystals in a test-tube and warming them. Instead of 
iodine, osmic acid or its vapour may be used, but then manipulation is 
extremely difficult, not to say unsatisfactory. 
By this method the objects are fixed at once, and then the iodine is 
removed by heating the preparation up to about 40° for 2-3 minutes. 
It is sometimes necessary to add a drop of distilled water during the 
evaporation of the iodine. The cover-glass, with the moist side still 
uppermost, is then put on a piece of elder-pith, about 3 mm. thick, and 
with a diameter rather less than the cover-glass. This, in its turn, rests 
upon a slide (Giessen size), 
which is placed in a glass Fig. 59. 
capsule, the sides of which are 
about 2 cm. high. The slide 
does not lie on the bottom of 
the capsule, but is placed on 
a sort of little stool made of 
metal (see fig. 59). 
To the preparation is 
added a drop of 20 per cent, 
alcohol and absolute alcohol 
in the capsule, the layer reach- 
ing half-way up the stool. 
The capsule is covered over 
and sealed up with vaselin. The vessel must be kept at an equal and 
moderate temperature, and not exposed to the sunlight. In a few hours 
the alcohol will have acted sufficiently upon the preparation. It is then 
removed and covered with a drop of collodion, or a solution of celloidin. 
When the celloidin has set a little, it is immersed in 80 per cent, 
spirit, wherein it becomes firmly set in about two minutes, so that the 
preparation may now be placed in any staining solution without fear of 
damage, dhe celloidin solution must be quite thin; the author uses 
the commercial solution diluted with six to ten parts of a mixture of 
equal parts of alcohol and ether. 
* Zeitschr. f. Wiss. Mikr., vii. (1890) pp. 13-1G (1 fig.). 
