Use of Gelatine in Bacteriology.
Gelatine fit for this purpose must be clear and bright, fairly neutral and of high gelatinizing power. For bacteriological purposes a 10 to 20 per cent. solution of gelatine in sheets or strips is made with meat broth, and this nutrient jelly, which is obtained by clarification with albumen perfectly clear and bright, forms a most useful medium for the cultivation of micro-organisms.
Artificial Silk from Gelatine.
For the production of textile threads, Millar utilizes the property of gelatine solution mixed with potassium dichromate, becoming insoluble on exposure to light. For this purpose a clear solution of gelatine is mixed with solution of potassium dichromate in the proportion of 100 parts of gelatine to 2 or 2½ parts of potassium dichromate. The fluid should be of such a consistency as to allow of its being drawn out into fine threads which on exposure to light become insoluble. Silk woven from such threads is equal in appearance to the genuine article but of course is not so strong. It is affected by moisture, becoming limp, but regains its normal strength when dry.[3]
[3] For further information on this interesting subject, the reader is referred to “Cellulose and Cellulose Products.” By Dr. Joseph Bersch. Henry Carey Baird & Co., Philadelphia, 1904.
[CHAPTER X.]
ISINGLASS AND ITS SUBSTITUTES.
Isinglass is obtained from the air-bladder or sound, as it is sometimes termed, of different kinds of fishes, especially of the sturgeon, species Acipenser. It is used for culinary purposes, fining beer and other liquids, for making court-plaster and stiffening silk, though a good quality of gelatine is practically equal to it for these purposes. A good quality of isinglass should be pure white, semi-transparent, dry and horny in texture, and free from odor. It should dissolve in water of 95° to 122° F., without leaving any residue, and, on cooling, should yield an almost colorless jelly. From gelatine imitations it is distinguished by soaking it in warm water and examining under the microscope, when true isinglass will show a net-work of long, curling fibres, while gelatine will be simply hyaline. Isinglass is often imitated with the intestinal membranes of the calf and of the sheep. This spurious article may be readily recognized because it does not exhibit, like isinglass, a sort of shining appearance when held before the eye and daylight, and because, although inodorous, it has a saltish flavor. If it be torn asunder it will be observed that it may be rent in all directions, while true isinglass cannot be divided otherwise than in the direction of its fibres. If a piece of artificial isinglass be macerated in water it swells, but instead of retaining its shape as is the case with the genuine article, it becomes divided into several pieces, forming a sort of curdy precipitate; and if treated with boiling water, about one-third of its weight is left in an insoluble state, and the liquor does not form a good jelly. Isinglass is frequently adulterated with gelatine, which is inserted between the leaves and rolled up with it. The best indication of this adulteration is the amount of ash; isinglass yields only 0.9 per cent., while gelatine yields 4 per cent., and adulterated isinglass 1.5 per cent. or more.
1. Russian isinglass. Russia produces the best and most isinglass. It is chiefly obtained from several varieties of sturgeon, species Acipenser, which inhabit the Caspian and Black Seas, and their tributary rivers. The Acipenser Gueldenstaedtii, Br. yields the finest, best and whitest isinglass. It is known by the name of Patriarch, and consists of small horseshoe-shaped pieces rolled tightly together. It is quite scarce and expensive. When the bladder is merely dried in sheets, it constitutes leaf isinglass. When several bladders are put together and folded before they are completely dry, they form book isinglass. Each bladder may also be rolled up and folded around a few pegs in the form of a horseshoe, heart, or lyre, in which shape it is dried. The latter is the staple isinglass, which, according to its dimensions, is again divided into long and short staple.
Long staple isinglass of fine quality is produced in the Oural. It is imported in loose leaves, and at times it is twisted like ropes, this kind being preferred, as it is inferior in quality only to Patriarch.