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cements, microscopical

Значение термина cements, microscopical в knolik

cements, microscopical - Cements, Microscopical
cements, microscopical - A cement invented by Dr. J. G. Hunt, is prepared as follows: Take dammar gum, any quantity, and dissolve it in benzole; after obtaining a solution just thick enough to drop readily from the brush, add enough of the finest dry oxide of zinc previously triturated in a mortar with a small quantity of benzole, until the solution becomes white when thoroughly stirred. If not too much zinc has been added, the solution will drop quickly from the brush, flow readily, and dry quick enough for convenient work. It will adhere, if worked properly, when the cell cover is pressed down, even when glycerine is used for the preservative medium; keep in an alcohol-lamp bottle with a tight lid and secure the brush for applying the cement in the lid of the bottle. Its advantages lie in the circumstance that the glass cover can be placed upon the ring of it whilst still fresh and soft, and that in drying it adheres to both cover and slide, so as to form a joint between them of the width of the ring of cement, and not, as with asphaltum, gold-size, etc., simply at the edge and upon the outside of the cover. The method of mounting with it is as follows: A ring of any desired size is made by means of an ordinary Shad-bolt's turntable, upon a slide, which is then placed to one side to dry; when required for use, the specimen, cover, etc., being all prepared and ready, the slide is again placed upon the turntable, and a new ring of cement put directly upon the old one. The specimen is immediately placed within the cell thus formed, and the requisite quantity of carbolated water placed upon it. The cover which must be large enough to entirely or nearly cover the cement ring, is now picked up with the forceps, the under side being moistened by the breath to prevent adhesion of air-bubbles, and carefully placed in position. It is now to be carefully and equably pressed down with some force; by this any superfluous water is squeezed out, and the cover is forced down into the cement, which rises as a little ring around its edge. The pressure is best made with a stiff needle, at first on the centre, and then upon the edges of the cover, which may finally be made slowly to revolve beneath the needle point. The slide may then be put aside to dry; or, better, an outside ring of the cement put over its edge in the usual way. If time be an object, and only a shallow cell be required, the first ring of cement may be dispensed with, and the whole mounting be done in a few minutes. According to Dr. L. Heydenreich of St. Petersburg, the best coverglass should be: (a) Absolutely hermetic, and should not, under any circumstances, require renewal every year. Two or three coats of the cement, applied at short intervals after an object is mounted, should permanently secure and preserve the object. (b) It should be as hard as glass, or, if possible, harder, (c) It should not crack nor become detached, and should be so solidly adherent as to be less likely to break than the glass to which it is attached. And (d) It should be insoluble in water or glycerine, or in any liquid used as an immersion medium with objectives. Notwithstanding the large number of cover-glass cements already known and in use, he thinks another should be sought for - one which shall conform to the foregoing requirements. We have commercial varnishes, which are very hard and durable. Some of them, used in the finishing of carriages, are found, after the lapse of a year, to be in the same condition as when first applied. The varnish used on tin pans in albumen factories remains unchanged for a year, although subjected daily, for many hours, to a temperature of 100° R. (257° F.). These and similar varnishes are made of resins, copal, or amber. Of all resins, amber and some kinds of copal are the hardest. Copal varnish is both hard and elastic; amber varnish is harder than copal, but not so elastic, and is, consequently, more brittle: hence, for a cover-glass cement, a mixture composed of both should be used. Only the best and clearest kinds of amber (the opaque pieces contain various kinds of minerals), and only the hardest kind of copal - that is, the East-India or Zanzibar copal - should be selected for cover-glass cements. Zanzibar copal is taken from the earth in flat, disc-shaped pieces, varying in dimensions from the size of a pea to the size of the human hand; is colourless, yellow, or of a dark red-brown colour, and transparent; the surface, rough. Bombay copal comes in larger pieces, is of a yellowish-red colour, has, when broken, a smooth, glassy surface, and is but very slightly inferior in quality to the copal of Zanzibar. Sierra-Leone copal comes in small, ball-shaped pieces, about 1 in. in diameter, or in pieces resembling drops in shape. All the other kinds are softer than those just described. The best solvent for resin, and the one which possesses the most adhesive quality, is linseed-oil varnish, made of pure, old linseed-oil. Neither alcohol, ether, chloroform, nor any other quickly evaporating menstruum should be used. In order to hasten dessication of the resin, and to obtain for the cement the proper consistency, an ethereal oil which, upon drying, will leave a surface perfectly even, should be added to the mixture; and oil of lavender, either alone or mixed with linseed-oil varnish, is suitable for these purposes. The resins being thus dissolved in linseed-oil varnish until the solution attains the consistency of syrup, oil of lavender should be added until the mixture becomes thin enough to use in mounting microscopical objects - and the cement is finished. The property of adhering to glass is increased in the cement by adding to it a small quantity of cinnabar; but such addition causes it to dry less rapidly. In a week from the time of using it the cement becomes dry, and so firm that the finger-nail will make but a slight impression on it. For months it remains in this condition. At the expiration of a year, it is very hard, and has a glassy surface.

So much for the component parts. The preparation of this cement being somewhat difficult it would perhaps be advantageous to buy the varnishes ready made, and then proceed as follows: Taking equal parts of the best, clearest, and hardest amber-varnish and copal-varnish, mix them and heat until all the turpentine has disappeared. This will require a temperature of 100° to 150° C. (257°-370° Fahr.). As soon as all the turpentine has evaporated, remove the dish from the flame, allow it to cool somewhat, and then add oil of lavender to the liquid in proportion of ½ to 1; mix well, and allow the entire mass to cool thoroughly. The process is terminated by adding from 20 per cent, to 40 per cent, of artificial cinnabar (rosin with cinnabar), which should be very carefully and thoroughly rubbed in. The best method for rubbing in the cinnabar is that employed in the preparation of fine oil-paints. Should the cement when finished be too thick for use, as much oil of lavender as will give the required fluidity may be added. The component parts and their proportions would then be as follows:

Amber	25 parts.
Copal	25 parts.
Linseed-oil varnish	50 parts.
Oil of lavender	50-60 parts.
Artificial cinnabar	40-60 parts.

Dr. Heydenreich continues his article by describing the manner in which the cement should be applied, but as his method is the same as that employed in the use of Canada balsam and other cover-glass cements, and, consequently, familiar to all microscopists, it is not necessary to make a note of it. However, he advises, in order to secure a perfect mount, that a second ring be made after the first or second week from the time of mounting; and a third, after the first or second month; each additional ring to be slightly wider than the preceding one. (See canada balsam.)

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