MICRO-CHEMICAL REAGENTS. 
In the course of the preceding representation the chemical 
detection of one or another substance has already been alluded 
to, and, indeed, the treatment of microscopic sections with 
suitable reagents affords many valuable disclosures. As in all 
other cases, definite answers are obtainable, when systematic 
and accurately formulated questions are propounded. For 
this purpose, chemical reagents! are of service, among which 
the following may be designated as especially important: 
1. Chromic Acid (free from sulphuric acid) dissolved in 100 
parts of water. This is adapted in general for the purpose of 
loosening composite, thickened cell-walls and constituent bodies, 
whereby the finer relations of structure are very often made 
evident, since chromic acid is also capable of clearing up 
darkly colored cell-walls, and, on the other hand separates the 
layers and finer membranes, thus bringing them more clearly to 
view. By means of this acid, starch granules are completely 
separated into lamine, the strata of the Cinchona bast-fibres 
(page 155, Fig. 73) separated from each other and lignified mem- 
branes @issolved, while suberized membranes are rendered more 
clear. 
When employed in a more concentrated form, or allowed to 
act for a longer time, chromic acid destroys the cell-walls. Its 
application, therefore, requires continual observation of the sec- 
tidns which are treated therewith, in order to completely survey 
the result of the phenomena. 
‘Compare Poulsen’s ‘“ Botanische Microchemie,” Cassel, 1881; and 
Behrens’ ‘‘ Hilfsbuch zur Ausfihrung microscopischer Untersuchun- 
_ gen,” Braunschweig, 1883. The American editions of these two works 
are noticed on page 49; in both of them are contained numerous refer- 
| _ ences to the literature of the subject. 
