C.—Sample (2) is next examined under reagents—iodine solution and sulphuric acid diluted with glycerine. The former is prepared as follows :— 1 grm. pure iodide of potassium is dissolved in 100 grm. distilled water, and an excess of iodide is added, to ensure constant saturation. It is kept in glass-stoppered bottles, always containing a few pieces of the iodide. The solution is liable to change at the end of a few months, and must then he renewed. The dilute sulphuric acid is thus prepared :-2 volumes of pure concentrated glycerine (Price's) are mixed with 1 volume of distilled water in a flask ; the latter is plunged into cold water to the level of its contents, and 3 volumes sulphuric acid, sp. gr. 1.845, are added with constant agitation. This solution also under goes gradual change by absorption of moisture, when a slight addition of acid becomes necessary.
Some thoroughly dry fibres, in a complete state of division, are selected from the sample, and briskly rubbed between the fingers ; they are then placed on a glass plate on the object-holder, and covered by a few drops of the iodine solution. The latter is allowed to thoroughly penetrate the fibres, and the excess is removed by blotting-paper. The test is then covered with a small piece of glass, along one edge of which are poured a few drops of the sulphuric acid preparation. The liquid penetrates between the two glasses, and advances towards the other side, where slips of blotting paper are placed to absorb it. The current is maintained for a little while, by occasionally adding a few drops of the acid, and renewing the blotting-paper. The result of this operation soon becomes evident in the distinct coloration of the fibres. Wherever cellulose is present, it assumes a blue or violet tint ; where the cellulose is lignified, or penetrated by foreign matters, it becomes yellow. This tint, which varies from bright-yellow to brownish-yellow, appears also in the fragments of tissue adhering to the fibres, and iu the matters occasionally found in their interior cavity. The coloration must be clear and pronounced, and the fibres must be in perfectly sound condition ; when the results are imperfect, the sulphuric acid preparation may need strengthening.. The blue colour will disappear in a few hours, and the yellow will not last beyond a day or two. The reagents occasionally disclose striations or transverse lines of deeper tint, generally arising from folds in the fibre, which afford additional characteristics.
D.—Sample (3) is divided into transverse sections, perpendicular to the axis of the fibre; these furnish the most precise indications of the form, structure, and thickness of the walls, of the fibres.
Sufficient fibres are taken to form a bundle about as thick as a goose-quill. About 1 in. of this is cut off, and tied in the middle by a thread. One end of the bundle is placed in thick glue, and then the other end, and both are pressed between the fingers, to ensure the close adhesion of the fibres, and to express the superfluous glue. The bundle is then hung up to dry, an operation requiring at least 12 hours in summer, and 24 or more in winter. When the mass is firm enough to bear cutting, it is divided by a razor into very thin sections, which give the best results when they exhibit the consistence of wax. Several examples should be taken from different parts of the sample, and especially from both ends of the fibres.
E.—The sections are next examined in one of the neutral liquids before prescribed. If the glue does not dissolve in it, recourse may be had to boiling in distilled water. After drying between blotting-paper, the sample is submitted to the microscope, while lying in the neutral liquid. The fibres are occasionally flattened, and present an oblong section, which should be measured both ways.
F.—The application of reagents follows. Into 2 or 3 drops of the iodine solution, is put some powder or fragments of the glued sample. In cold weather, the glass plate should be warmed, to facilitate the solution of the glue. The iodine solution should be absorbed gradually, and the powder should be spread as equally as possible. The excess of iodine is removed by blotting-paper, the sample is covered by a second glass, and the sulphuric acid is introduced, as in previous tests. Notes are made of the exterior form of the sections, the thickness of their walls, and the form of the central cavity. Some fibres appear to be composed of a compact and homogeneous substance, others have concentric coats, which assume varying shades of blue ; some present fissures or perpendicular striations on both the interior and exterior surfaces, and which seem to radiate from the centre; some contain a yellow granular matter, while others are empty ; and some are bordered with a yellow line.
G.—It is often useful to examine the fibres as they exist in the plant, to determine their position, relative abundance, and nature. Sections are taken of the fibre-yielding portion of the plant, which must be either freshly cut, or soaked in water. These are placed to macerate for several hours in a mixture of glycerine and water, and are then treated with a few drops of concentrated glycerine, which penetrates them, and renders them transparent.