the solvent is added until a fairly thin fluid is obtained. Perhaps the best solvent of all is xylol. It requires nearly twice its volume of xylol. Filter through paper. The balsam should be kept in a "capped" bottle (fig. 48) instead of a

Chloroform

Dissolve the dammar in the turpentine, and filter; the mastic in the chloroform, and filter. Mix the two solutions and filter again.

There must be no moisture in the bottles, and the mixture must be kept in 'capped" bottles, else the chloroform will evaporate.

Xylol-Balsam.-Dry ordinary Canada balsam in a sand-bath, to drive off all the moisture, and until it becomes vitreous. If it be spread out in a thin layer in a tin vessel, this is usually accomplished in two hours or so, but the balsam must not be overheated or change its colour and become brown. Dissolve the dried balsam in an equal volume of xylol. Perhaps this is the best form of balsam to use.

Balsam, when prepared, should be kept in a glass bottle with a ground-glass cap.

To Place a Section on a Slide.-By far. the most convenient method is to place the section in a basin of water. Hold the slide perpendicularly by the edges in the left hand, plunge the slide into the water until it is about three-fourths immersed, and with a mounted needle pull the section on to the slide, and at the same moment raise the latter out of the water. The section adheres to the glass, and if it be folded at one end, dip this end in the water, when it floats out quite flat. Do not attempt to spread out the folds on the slide by means of a needle.

Hold the slide vertically to allow the water to drain off, and remove with a rag or well-washed cloth the remainder of the water close up to the section.

It may be stained on the slide. After the staining is complete, remove the surplus dye by means of bibulous paper, taking care, however, that the section itself does not adhere to the absorbent

paper.

If the section is to be mounted in glycerine or Farrant's solution, add a drop of either of these reagents and apply a cover-glass.

If the section is to be mounted in balsam, remove as much as possible of the surplus water or dye, as the case may be, and pour methylated spirit upon the section. Allow it to remain on the section for a minute or so, and drain it off at one end of the slide. Apply fresh methylated spirit again, and finally absolute alcohol. This is done to secure complete dehydration. The frequent and prolonged application of strong spirit removes all the water.

Remove as much of the spirit as possible, but do not allow the section to dry. It is now ready to be cleared up.

With a brush insinuate a drop of the clarifying reagent-cloveoil or xylol-under one corner of the section, and allow the xylol to flow under the whole of the section. It will gradually diffuse into the tissue; and if the process be watched under the microscope with a low power, the section will be seen to become gradually more transparent, while the spirit will be seen as fine globules driven out into the essential oil. The success of the process depends on complete removal of the water by spirit, and the complete removal of the latter by the essential oil used as the clarifying reagent. If any opacity remains, and it looks milky or like an emulsion, there has been either water or spirit, or both, left in the section.

More of the essential oil is placed on the section, so that it is completely bathed in it and rendered quite clear by it. Pour off the superfluous oil, remove the surplus close up to the edge of the section, add a drop of balsam, apply a cover-glass, and the process. is complete.

In some cases it is convenient to put the drop of balsam on the cover-glass, and then to invert this on the clarified preparation. In all cases where it is directed to mount in balsam, this process must be gone through, viz.

(1.) Stain the section.

(2.) Wash it in water.

(3.) Treat the section with strong alcohol (96 per cent.) to remove water (3-5 minutes).

(4.) Absolute alcohol (3-5 minutes).

(5.) Clarify with an essential oil to remove all the alcohol.

(6.) If the section be not on a slide already, place it on a slide by means of a lifter. Remove surplus oil with blotting-paper.

(7.) Add balsam, cover the section with a cover-glass.

(8.) If desired, the hardening of the balsam may be hastened by gently warming the preparation on a water-bath.

Sometimes it is not convenient to stain, dehydrate, and clarify a section on a slide. In this case the sections are stained, dehydrated,

and clarified in watch-glasses, the sections being transferred from one fluid to the other, and finally to the slide by means of a "lifter" (p. 3).

Sometimes the one method is adopted, sometimes the other.

To Clean a Microscopic Preparation. Any excess of balsam round the edge of a preparation may be moved with a cloth dipped in benzol.

In the case of a preparation mounted in glycerine, any excess of the latter must be removed with great care, otherwise the cement will not adhere to the glass.

With preparations mounted in Farrant's solution, leave them in an airy dry place for ten days or longer; this gives the medium. time to harden at the edges, and fixes the cover-glass pretty firmly to the slide. Place the slide in a basin of water, and with a camel's-hair brush brush away from the edge of the cover-glass every trace of the medium. There is no fear of disturbing the cover-glass. Lave the slide in fresh water, and then wipe it thoroughly dry. It is better to wash a number of slides at a

time.

To cement or "Ring" the Specimens. Balsam Preparations need not be touched. They keep perfectly without being covered in by coating the edge of the cover-glass with an adhesive and resistant cement. If it be desired to cement them, a thin coating of Hollis's glue must first be applied, and after it is dry the cement is laid on as directed for preparations mounted in Farrant's solution.

To Ring a Slide. The slide should be fixed on a turntable, the centre of the circular cover-glass corresponding to the centre of the brass disc of the table. The slide is

fixed in position by means of two brass clips (figs. 49, a, b, 50).

For Farrant's preparations or glycerine preparations, lay on a ring of white zinc cement with a goat's-hair brush.

The disc

FIG. 50.-Showing how slide is to be centred on the Turntable.

is made to revolve with the fore-finger of the left hand, but not too quickly, and a coating of the cement is laid on evenly. The forefinger is applied to the smaller disc (c).

The turntable should be heavy and mounted on a pin-point centre-piece. The brushes must not be too large, and should be washed immediately after use in the same fluid as is used to dissolve the cement. Thus, for zinc-white the brush is to be washed in benzol or xylol, and for gold-size in turpentine, and for Farrant's solution in water.

White Zinc Cement.-Dissolve 3 oz. of dammar in 3 oz. of benzol, and add 200 grains of finely-ground oxide of zinc. Mix the whole thoroughly, and strain through several folds of muslin. It is perhaps more convenient to purchase the cement.

Mounting Block. It is important that the section be placed in the centre of the slide. As a guide for this purpose, cut a piece of paper the size of the slide, and draw diagonal lines from corner to corner of it; they will intersect in the centre. Or the piece of paper may be gummed by means of Hollis's glue between two slides.

XV. INJECTING BLOOD-VESSELS AND

GLAND-TUBES.

Transparent Injection Masses.-At the present time, histologists use transparent injections, consisting of a vehicle-which may be water, glycerine, or gelatine-and a colouring matter. Most commonly gelatine is used as a vehicle. The colouring matter of most red injections is carmine. In this case, the secret is to have the mass as neutral as possible.

1. Carter's Carmine Injection.

Carmine

Strong solution of ammonia

Glacial acetic acid

Solution of gelatine (1 to 6 water)
Distilled water

I dr.

2 fl. drs. 86 mins.

2 OZ.

1,,

Rub up the carmine with a little water in a mortar, add the remainder of the water, and then add the ammonia, and stir until the carmine is dissolved. Add the glacial acetic drop by drop, stirring thoroughly. Add the gelatine solution. and stir briskly.

2. Ranvier's Method. --The following method yields excellent results. Mix 2-5 grms. of pure carmine with a little distilled water in a stoppered bottle, and add ammonia solution, drop by drop, until the carmine is dissolved, which occurs when the liquid becomes transparent. Shake up the liquid to get it homogeneous. Weigh 5 grms. of dry Paris gelatine (Coignet's), and place it in distilled water for one hour. At the end of this time it is swollen up and soft. Remove it from the water, wash it in water, and

place it in a beaker in a water-bath. When the gelatine is dissolved by the water which it has absorbed, add to it-stirring vigorouslythe solution of carmine, which yields an ammoniacal solution of carmine in gelatine.

When the carmine mixture is on the water-bath make a solution of

Distilled water.

Glacial acetic acid

2 parts.
I part.

Pour the acid drop by drop into the mass, stirring thoroughly all the time with a glass rod. The acid is to neutralise the excess of ammonia. This requires great attention. It is by the odour that one recognises when the fluid is neutralised. As the acid is added the ammoniacal odour diminishes, and there is at last a faint acid odour. This is the moment to stop adding the acid. Towards the end of the operation it is best to dilute the acid somewhat.

Filter the mass through new flannel.

3. Carmine Gelatine Mass (Carter's) (Fearnley's method).

Cut up the gelatine into small pieces and place it in 50 cc. of the water to swell up, i.e., for four or five hours. Rub up the carmine in a mortar with a little water and add the ammonia. Let it stand for two hours and then pour it into a bottle, rinsing the mortar with the remainder of the water. Place the swollenup gelatine, and any remaining water unabsorbed by it, on a water-bath until it melts. To the dark purple carmine fluid add the acid (a few drops at a time), mixing the two thoroughly, and as soon as the fluid changes to a crimson stop adding the acid. To the melted gelatine add the crimson carmine little by little and keep stirring all the time.

This mass may be kept in a cool place for a long time if its surface be covered with methylated spirit. Before using it, dissolve it on a water-bath, and filter it through fine flannel wrung out of hot water. The best gelatine to use is French gelatine-Coignet's.

4. Blue Mass.---The mass is made with gelatine coloured with soluble Prussian blue or Brücke's blue. It is very difficult to obtain a pure sample of Brücke's blue, but this can now be had from Dr. Grübler of Leipzig. Use a saturated watery solution of Brücke's blue.

Weigh 5 grms. of gelatine, and treat it exactly as described