MICRO-CHEMICAL REAGENTS. 277 
of the specific gravity 1.405 is diluted with 10 times its weight 
of water, and the sections allowed to macerate for some time in 
the liquid. If aleohol be employed for diluting the solution, 
somewhat different reactions are usually obtained, and, upon the 
subsequent addition of lime-water, still further changes of color 
appear. The dilute solution of ferric chloride is decomposed by 
long keeping (as a result of dissociation). Only the officinal 
solution should, therefore, be kept ready prepared. 
The dilute solution of ferric chloride serves chiefly for the 
recognition of tannic matters, which are thereby colored either 
green or blue. It is expedient to discriminate between these 
two classes of colorations, though this is frequently difficult, 
owing to the appearance of transition colors in consequence of 
several tannic matters being usually present at the same time. 
This assumption of the simultaneous presence of several differ- 
ent tannic matters is supported by the observation that the 
coloration first produced by very small quantities of solution of 
ferrous sulphate in cells containing tannin is often changed by 
the further addition of ferric chloride. The behavior of pyro- 
catechin, quercitrin, and rutin to iron salts may here also be | 
called to mind. 
Instead of ferric chloride, ferric sulphate or ferric acetate may 
also be employed. 
32. Mereurous Nitrate, known also by the name of “ Mil- | 
lon’s reagent.” One part of mercury is dissolved, without 
heat, in 1 part of fuming nitric acid, and the solution diluted 
with 2 parts of water. This liquid imparts a red color to pro- 
tein substances, though only when the latter are present in 
considerable amount. The striping of the membranes is ren- 
dered clearer by Millon’s reagent. On account of its strongly 
acid reaction, care must be taken not to have it come in contact. 
with the microscope. 
33, Aniline Sulphate, in aqueous, or better, alcoholic solu- _ 
tion, colors all lignified membranes yellow, especially after the oe : 
addition of sulphuric or hydrochloric acid. 
34. Phloroglucin is a still more delicate reagent for lignifi- 
cation. The sections are thoroughly moistened with hydro~ 
