Supposing the precipitation to be complete, or very nearly so, shake vigorously for at least half an hour, and allow it to stand until quite cold. The shaking must then be renewed for some time, and the bottle filled up with pure water.
After allowing the precipitate to settle a day, draw off the clear supernatant fluid with a syphon. Repeat the washing until the clear liquid gives little or no precipitate with chloride of barium. So much water must be left with the colour at last that it shall measure forty fluid ounces.
For the injecting fluid take twenty-four ounces of the above coloured liquid, and three ounces of good gelatine. Allow these to remain together twelve hours, and then dissolve by the heat of a water bath; after which it should be strained through fine muslin.
As this injecting fluid contains gelatine, the hot water, and other contrivances mentioned in a former part of the chapter, will be necessary here also, but no peculiar treatment will be required.
CHAPTER VII.
MISCELLANEOUS.
It must be evident to all readers that there are various objects of interest to the microscopist which cannot be properly placed amongst any of the forementioned classes, but must not be omitted in such a guide as this professes to be. Of these may be mentioned the circulation of the blood in various animals, the rotary motion of the fluid in many plants, the best means of taking minute photographs, &c. &c.
Perhaps the most interesting of these objects is the circulation of the blood through the finer vessels of various parts of the animals made use of for these purposes, which parts, it is evident, must be very transparent to afford a perfect view of this phenomenon. The web of the frog’s foot is very frequently made use of, but requires a certain arrangement, which we will now describe. A piece of thin wood (Dr. Carpenter recommends cork) is taken, about eight inches long and three wide; about an inch from one end is cut a hole, half or three-quarters of an inch in diameter. The body of the frog is then placed in a wet bag, or wrapped in wet calico, whilst the hind-foot projects; the whole is then laid upon the piece of wood so that the foot, which is left free, may be extended over the hole. The web must then be spread out, and secured either by threads to small pins on the wood, or the pins must be driven through the web into the wood, and so kept in position. A few bands of tape must be passed round the body, the leg, and the wood, to prevent any disarrangement arising from the animal starting, &c. Care must be taken that the tape is not too tight, else the circulation will be very slow or altogether stopped. The wood must now be fixed upon the stage, with the aperture under the object-glass: this is sometimes done by simply binding it, or a spring is fixed so as to accomplish the same object without so much trouble. With a half-inch power the blood may now be seen to flow very distinctly. The frog may be used for hours if care is taken to prevent the web from becoming dry, by wetting it with a little water from time to time. The piece of wood or cork upon which the frog is laid is often made to give place to the “frog-plates,” supplied by opticians. These are made of brass, somewhat resembling the piece of wood above recommended, but each maker’s pattern differs according to his own taste.
The tongue of the frog is also sometimes used for the purpose of showing the circulation of the blood, which is done in the following manner:—The body is wrapped with the calico, and made fast to the plate as before, only the mouth of the frog is brought to the opening. The tongue is then gently drawn out of the mouth and pinned down over the aperture, when the circulation will be well shown. But, as Dr. Carpenter observes, the cruelty of this mode of treatment is so repulsive that it is unjustifiable.
Tadpoles of the frog (which, of course, are only obtainable in their season) are good subjects for showing the circulation of the blood. They are best suited for the microscope when about one inch long. The tadpoles of the newt and toad also are equally suitable. They may be placed in a very shallow glass trough with a little water, and a narrow band of linen bound lightly round in some part not required for examination, to keep them from moving; or they may be laid upon a glass plate with a drop or two of water, and a thin glass covering lightly bound upon it. Dr. Carpenter, however, places them first in cold water, gradually adding warm until the whole becomes about 100°, when the tadpole becomes rigid, whilst the circulation is still maintained. I have not, however, found this necessary, the thin glass accomplishing all that is desired. The tail is generally the most transparent, and shows the circulation best; but in some of the newt larvæ the blood may be traced down to the very extremities if they are not too old. Mr. Whitney places the tadpole upon its back, by which means the heart and other internal arrangements may be seen.