made by dissolving gum dammar in benzole, and adding about one-third of gold size: it dries very readily, and is especially useful when mounting objects in fluid, taking care that no moisture extends beyond the covering glass, which would prevent the complete adhesion of the cement. In those cases where glycerine is employed as the mounting medium, a ring of liquid glue put round the cover first, and when that is dry, a second coat of gum dammar will keep the cover very secure, and no leakage take place. LIQUID GLUE is another of these cements, which is made by dissolving gum shellac in naphtha in such quantity that it may be of the required consistency. This cement appears to me almost worthless in ordinary work, as its adherence can never be relied upon; but it is so often used and recommended that an enumeration of cements might be deemed incomplete without it. Even when employed simply for varnishing the outside of the glass covers, for appearance's sake alone, it invariably chips. Where, however, oil is used as a preservative liquid, it serves very well to attach the thin glass; but when this is accomplished, another varnish less liable to chip must always be laid upon it. Chapter V.) Yet it makes excellent cells. (See BLACK JAPAN.-This is prepared from oil of turpentine, linseed oil, amber, gum anime, and asphalt. It is troublesome to make, and therefore it is much better to procure it at the shops. It is a really good cement, and serves very well to make shallow cells for liquids, as will be described in Chapter IV. The finished cell should be exposed for a short time to the heat of what is usually termed a "cool oven." This renders it very durable, and many very careful manipulators use it for their preparations. ELECTRICAL CEMENT.-This will be found very good for some purposes hereinafter described. To make it, melt together 5 parts of resin. It must be used whilst hot, and as long as it retains even slight warmth can be readily moulded into any form. It is often employed in making shallow cells for liquids, as before mentioned. GUM-WATER is an article which nobody should ever be without; but labels, or indeed any substance, affixed to glass with common gum, are so liable to leave it spontaneously, especially when kept very dry, that I have lately added five or six drops of glycerine to an ounce of the gum solution. This addition has rendered it very trustworthy even on glass, and now I never use it without. Ten grains of moist sugar to each ounce of gum solution will also answer equally well. This solution cannot be kept long without undergoing fermentation, to prevent which the addition of a small quantity of any essential oil (as oil of cloves, &c.), or one-fourth of its volume of alcohol, may be made, which will not interfere in any way with its use. There is what is sometimes termed an extra adhesive gum-water, which is made with the addition of isinglass, thus-Dissolve two drachms of isinglass in four ounces of distilled vinegar; add as much gum arabic as will give it the required consistency. This will keep very well, but is apt to become thinner, when a little more gum may be added... I may here mention that Messrs. Marion have lately brought out a cement for the purpose of mounting photographs, which is very adhesive, even to glass. I find it useful in all cases where certainty is requisite; as gummed paper is liable in a dry place to curl from the slides, as before mentioned. All these, except one or two, are liquid, and must be kept in stoppered bottles, or, at least, as free from the action of the air as possible. When any two substances are to be united firmly, I have termed the medium employed "a cement;" but often the appearance of the slides is thought to be improved by drawing a coloured ring upon them, extending partly on the cover and partly on the slide, hiding the junction of the two. The medium used in these cases I term ▲ VARNISH, and hereinafter mention one or two. Of course, the tenacity is not required to be so perfect as in the cements. SEALING-WAX VARNISH is prepared by coarsely powdering sealing-wax, and adding spirits of wine; it is then digested at a gentle heat to the required thickness. This is very frequently used to finish the slides, as before mentioned, and can easily be made of any colour by employing different kinds of sealing-wax; but is very liable to chip and leave the glass. The best qualities, however, will be less liable. BLACK VARNISH-Is readily prepared by adding a small quantity of lampblack to gold-size and mixing intimately. Dr. Carpenter recommends this as a good finishing varnish, drying quickly and being free from that brittleness which renders some of the others almost worthless; but it should not be used in the first process when mounting objects in fluid. Amongst these different cements and varnishes I worked a long time without coming to any decision as to their comparative qualities, though making innumerable experiments. The harder kinds were continually cracking, and the softer possessed but little adhesive power. To find hardness and adhesiveness united was my object, and the following possesses these qualities in a great degree : India-rubber Dissolve the india-rubber in the naphtha, then add the asphaltum-if necessary, heat must be employed. Some scientific friends have complained that they have been unable to dissolve either the india-rubber or the asphaltum in mineral naphtha. The frequency with which I have seen this solution thoroughly accomplished convinces me that one of these things has occurred-either the indiarubber or the asphaltum has not been pure, or the naphtha has been wood instead of mineral. In the early photographic days every artist made a form of this varnish to use with glass positives, and I never heard a complaint of difficulty. This is often used by photographers as a black varnish for glass, and never cracks, whilst it is very adhesive. Dr. Carpenter, however, states that his experience has not been favourable to it; but I have used it in great quantities and have never found it to leave the glass in a single instance when used in the above proportions. The objections to it are, however, I think easily explained, when it is known that there are many kinds of pitch, &c., from coal, sold by the name of asphaltum, some of which are worthless in making a microscopic cement. When used for this purpose, the asphaltum must be genuine and of the best quality that can be bought. The above mixture serves a double purpose-to unite the cell to the slide, and also as a "finishing" varnish. But it is perhaps more convenient to have two bottles of this cement, one of which is thicker than common varnish, to use for uniting the cell, &c.; the other liquid enough to flow readily, which may be employed as a surface varnish in finishing the slides. The brushes or camel-hair pencils should always be cleaned after use; but with the asphalt varnish above mentioned it is sufficient to wipe off as carefully as possible the superfluous quantity which adheres to the pencil, as, when again used, the varnish will readily soften it; but, of course, it will be necessary to keep separate brushes for certain purposes. Here it may be observed that every object should be labelled with name and any other descriptive item as soon as mounted. There are many little differences in the methods of doing this. Some write with a diamond upon the slide itself; but this has the disadvantage of being not so easily seen. For this reason a small piece of paper is E usually affixed to one end of the slide, on which is written what is required. These labels may be bought of different colours and designs; but the most simple are quite as good, and very readily procured. Take a sheet of thin writing paper and brush over one side a strong solution of gum, with the addition of a few drops of glycerine, or grains of moist sugar, as above recommended; allow this to dry, and then with a common gun-punch stamp out the circles, which may be affixed to the slides by simply damping the gummed surface, taking care to write the required name, &c., upon it before damping it, or else allowing it to become perfectly dry first. There is one difficulty which a beginner often experiences in sorting and mounting certain specimens under the microscope, viz., the inversion of the objects; and it is often stated to be almost impossible to work without an erector. But this difficulty soon vanishes, the young student becoming used to working what at first seems in contradiction to his sight. Let it be understood, that in giving the description of those articles which are usually esteemed necessary in the various parts of microscopic manipulation, I do not mean to say that without many of these no work of any value can be done. There are, as all will allow, certain forms of apparatus which aid the operator considerably; but the cost may be too great for him. A little thought, however, will frequently overcome this difficulty, by enabling him to make, or get made, for himself, at a comparatively light expense, something which will accomplish all he desires. As an example of this, a friend of mine made what he terms his "universal stand," to carry various condensers, &c. &c., in the following way:-Take a steel or brass wire, threesixteenths or one-quarter inch thick and six or eight inches long; "tap" into a solid, or make rough and faster with melted lead into a hollow, ball. (The foot of a cabinet or work-box answers the purpose very well.) In the centre of a round piece of tough board, three inches in d'ameter, |