MARINE GLUE.--No cement is more useful or trustworthy for certain purposes than this. It is made in various proportions; but one really good mixture is--equal parts of india-rubber and gum shellac; these are dissolved in mineral naphtha with heat. It is, however, much better to get it from the opticians or others who keep it. It requires heat in the application, as will be explained in Chapter IV.; but it is soluble in few, if any, liquids used by the microscopist, and for that reason is serviceable in the manufacture of cells, &c. Where two pieces of glass are to be firmly cemented together, it is almost always employed; and in all glass troughs, plates with ledges, &c., the beginner may find examples of its use. GOLD SIZE.--This substance may always be procured at any colourman’s shop. The process of its preparation is long and tedious. It is, therefore, not necessary to describe it here. Dr. Carpenter says that it is very durable, and may be used with almost any preservative liquids, as it is acted upon by very few of them, turpentine being its only true solvent. If too thin, it may be exposed for awhile to the open air, which by evaporation gradually thickens it. Care must be taken, however, not to render it too thick, as it will then be useless. A small quantity should be kept on hand, as it is much more adhesive when _old_. 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’ 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. (See Chapter IV.) 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 make use of 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. 1 ” beeswax. 1 ” red ochre.[A] [A] Dr. Griffiths says that the addition of 2 parts of Canada balsam renders this cement much more adhesive to the glass.