a 20% solution of potassium iodide. In this diluted form, when applied to the sample, a reddish or violet coloration is imparted to cell contents having tannin.

Phenol-hydrochloric Acid is prepared by saturating concentrated hydrochloric acid with the purest crystallized carbolic acid. Wood fiber, or lignin, when treated with a drop of this reagent under the cover-glass, and exposed for half a minute to the direct sunlight, will be colored an intense green, which quickly fades.

Indol.-Several crystals of indol are freshly dissolved in warm water. Lignified cell walls assume a deep-red color, when the specimen to be examined is treated first with a drop of the indol reagent, and afterwards washed with dilute sulphuric acid, 1:4.

Millon's Reagent.-This is prepared by dissolving metallic mercury in its weight of concentrated nitric acid, and diluting with an equal volume of water. This reagent, which should be freshly prepared, is of use in testing for protein compounds, which turn brick red when treated with it, especially on gently warming the slide.

Tincture of Alkanna.-A 70 or 80% alcoholic extract of alkanna root, when kept in contact with resins, fixed oils, fats, or essential oils for a short time, stains these cell contents a lively red. The staining is hastened by the aid of heat. Essential oils and resins are soluble in strong alcohol, while fixed oils and fats are insoluble, hence the distinction between these classes of cell contents may be made by the application of alcohol to the alkanna-stained specimen.

Ferric Chloride, Ferric Acetate, or Ferric Sulphate, used in dilute aqueous solution, are all applicable as reagents for tannic acid, which, when present in appreciable amount, will be colored green or blue by either of these reagents.

B. Clarifying Reagents.-Many of the harder cellular tissues are too opaque for careful examination, and may be rendered transparent by clarifying or bleaching. The simplest and for many purposes the most satisfactory method for clearing the tissues is by boiling a water mount, replacing the water lost by evaporation. Proceeding in this manner, there is ordinarily no danger of the slide or cover-glass breaking; if the boiling is carried out without a cover-glass, the slide is almost sure to break. A portion of the powdered sample is either boiled with a drop of the reagent under the cover-glass or is allowed to soak for hours or even days in the reagent, using a drop of the same reagent as a medium for examination on the object-glass, instead of water. The clarifying reagents most commonly used are the following:

Chloral Hydrate.-A 60% solution.

Ammonia. Concentrated, or 28% ammonia is commonly used.

Potassium Hydroxide, used in various degrees of concentration, often in dilute solution, say 5%. This reagent, added to a water mount, causes swelling of the cell wall, and dissolves intercellular substances and protein. It bleaches most of the coloring matters, destroys the starch, and forms soluble soaps with the fats. Potassium hydroxide is also used in testing for suberin, which is extracted from corky tissue on boiling with the reagent, and appears as yellow drops.

Schultze's Macerating Reagent (concentrated nitric acid and chlorate of potassium) is best used by placing the powder or bit of tissue to be treated in a test-tube with an equal volume of potassium chlorate crystals, adding about 2 cc. of concentrated nitric acid, and warming the tube till bubbles are evolved freely, or until the necessary separation of cells is effected. The sample is then removed and washed with water.

By this treatment, bast and wood fibers as well as stone cells are readily separated from other tissues.

Cuprammonia (Schweitzer's Reagent).—This is prepared by adding slowly a solution of copper sulphate to an aqueous solution of sodium hydroxide, forming a precipitate of cupric hydroxide, which is separated by filtration, washed, and dissolved in concentrated ammonia. It should be freshly prepared, and is never fit for use unless it is capable of immediately dissolving cotton. Indeed its chief use is as a test for cellulose, which it readily dissolves. In observing this reaction under the microscope, the powdered specimen under the cover-glass should be only slightly damp before a drop of the fresh reagent is applied. The cell walls are seen to swell up and gradually become more and more indistinct, till they finally disappear.

Cuprammonia is also used as a test for pectose, which occurs in many cell walls, often intermixed with cellulose. When treated with this reagent, cellular tissue containing pectose is acted upon in such a manner that a delicate framework of cupric pectate is sometimes left behind, after the dissolution of the cellulose with which it is mingled.*

PHOTOMICROGRAPHY.

The photomicrograph serves as a simple means of keeping permanent records of unusual forms of adulteration encountered in the course of routine examination. Besides this, the photomicrograph has at times proved its usefulness as a means of evidence in court, showing as it does with faithfulness the presence of a contested adulterant. It is true * Poulsen, Botanical Micro-chemistry, p. 15.

that from an artistic and didactic standpoint the photomicrograph of a powdered sample is often disappointing, due to the fact that ordinarily much of the field is out of focus, unless a very simple homogeneous subject is photographed, as, for instance, starch. As compared with the carefully prepared drawing of a section, which shows minute details of structure, the photomicrograph portrays what happens to be in focus.

SUMMARY OF MICROCHEMICAL REACTIONS FOR IDENTIFYING CELLULAR TISSUE AND CELL CONTENTS. BASED ON BEHRENS'.*

* Microscopical Investigation of Vegetable Substances, page 356. + When treated with the reagent, suberin forms masses of ceric acid, soluble in ether, alcohol, or chloroform.

While the analyst examines microscopically the ordinary powdered spice, for example, he constantly moves with his hand the fine adjustmentscrew, bringing into focus different parts of the field successively. This

he does unconsciously, so that he does not realize how far from flat the field actually is till he undertakes to photograph it, when, as a rule, only a small portion is in good focus. It is therefore impossible in one photograph to show successfully many varied forms of tissue or cell contents in the powder, but separate photographs should be made as far as possible with only a little in each. Thus, for example, with a composite subject like powdered cassia bark, it would be very difficult to show starch, stone cells, and bast fibers in one field, all in equally good focus, and, for the best results only, one, or at most two, such varied groups of elements should be shown in one picture.

Appurtenances and Methods of Procedure. The temporary method of water-mounting employed by the analyst in routine examination presents many difficulties from a photographic point of view. The vibrating motion of the particles is very annoying, and some skill is required in using just the right amount of water, in avoiding air-bubbles, in waiting the requisite amount of time before exposing the plate for the vibratory motion to cease, and, on the other hand, avoiding too long delay, which would result in the evaporation of the water, and the consequent breaking up of the field. In the writer's experience, however, in spite of these difficulties, the water-mounting gives decidedly the clearest results, and, with patience on the part of the operator, it is in many ways the most desirable method of mounting for photographic purposes. It is in fact the method employed in making most of the photomicrographs of powdered specimens that appear in the plates at the end of this volume, though a few were mounted in glycerin jelly, and the starches for the polarized-light pictures in Canada balsam. The sections of tissues shown in the plates were mounted some in glycerin and others in glycerin jelly.

Experience has shown that two degrees of magnification well calculated to bring out the chief characteristics of the spices and their adul terants in a photomicrograph are 125 and 250 diameters. The starches, which are the most common of any one class of adulterants, vary very widely in the size of their granules. With these the larger magnification of 250 has been found satisfactory, while the general appearance of the composite ground-spice itself under the microscope, as well as that of such adulterants as ground bark, sawdust, chicory, pea hulls, and the like, is best shown with the lower power of 125.*

*The degrees of magnification adopted in the originals of most of the photomicrographs illustrated in the accompanying plates are accordingly 125 and 250, but in the process of lithographing, the photographs were slightly reduced, so that the actual scales in the reproduction are 110 and 220 respectively.

The object, mounted in the manner above described, is best examined when held in a mechanical stage, furnished with micrometer adjustments, in such a manner that a typical field may be selected and held in place long enough to photograph.

The Camera. This need not of necessity be complicated, but may consist simply of a horizontal wooden base on which the microscope

FIG 374.-A Convenient Photomicrographic Camera.

rests, and an upright board firmly secured to the base, carrying a frame for an interchangeable ground glass and plate-holder, with a rubber gauze skirt hanging from the frame, adapted to be gathered and tied about the top of the microscope-tube. Means are further provided, as by a slotted guide and screw, for adjusting the frame at any desired height on the upright board.*

A more convenient form of apparatus now employed by the writer is that shown in Figs. 37a and 376.

* Such a contrivance as this was employed in making some of the accompanying photomicrographs.