Cap cements are so called because they are used for fixing brass caps, stopcocks, etc, on glass apparatus. There are two kinds of cement in use for this purpose; one consists of resin and other matters, and is fusible by heat, so that it is easily applied, takes very little time to harden, and, if the glass should get broken, or if the brass work requires to be changed, it is very easy to separate the parts by the action of heat. When properly applied, this cement is perfectly air-tight, and is very strong. The only objection to it is that it is easily softened by heat, and therefore cannot be used for apparatus to which heat is to be applied. For air-pumps and other pneumatic apparatus, and similar purposes, it answers perfectly. The other cement consists of white or red lead ground in boiled oil, and applied either to the naked surfaces, or by spreading it on a cloth, which is then placed between the surfaces to be united. The advantage of this kind of cement is that it will stand any heat below 300° F. (149° C), and that it is steam and air tight. The objections are that it takes a long time to dry, and that when it has been used to unite pieces of apparatus, it is almost impossible to separate the parts without breaking the glass.

This may occasionally be effected, however, either by heating the joint very strongly, or by soaking in solution of caustic potash or soda. (1) Faraday's, or Electrical. Resin, 5 oz.; beeswax, 1 oz.; red ochre or Venetian red in powder, 1 oz. Dry the earth thoroughly on a stove at a temperature above 212° F. (100° C). Melt the wax and resin together, and stir in the powder by degrees. Stir until cold, lest the earthy matter settle to the bottom. Used for fastening brass work to glass tubes, flasks, etc. Faraday's directions for fastening caps to the ends of tubes or retorts are as follows: - "One is to be selected of such size as to admit the tube and allow a space for cement about the thickness of a card or a little more, but the cap should never be so small as itself to gripe the glass, or any larger than is necessary to allow room for cement to surround the glass. The cement should be heated to fluidity on the sand-bath, but not to a greater degree; the cap should be warmed over a candle or lamp until it is hot enough to melt cement, and then that part of its interior which is intended to come against the glass, viz. the side of the cylinder, should be covered with the hot cement, applied by a piece of stick.

The cap being then laid on its side by the sand-bath to keep it from cooling, the end of the tube or retort is next to be warmed, and a coat of cement applied on the exterior, over every part which is to come into juxtaposition with the cap, but the other parts are not to be unnecessarily soiled; so much cement is to be left adhering to the glass, that with what there is in the cap, there may be an excess above the quantity that can be retained between the glass and metal when the two are fitted together. When the cap, glass, and cement are all so warm that the latter is fluid or very soft, the cap is to be placed upon the tube, thrust into its right position, receiving a little rotary motion, at the same time to distribute the cement equally over all parts, and is afterwards to be set aside to cool. When this is well performed, the retort neck or tube should pass along until it is stopped by the inside of the shoulder; no cement should soil its interior or project within the cap, but it should fill every part between the glass and the cap to make a firm, tight junction, and project in a ring from the edge of the cap over the exterior of the glass. The superabundance is easily removed by a knife, and the annular surface left made smooth and tight by a hot wire passed rapidly over it.

If a piece of cement, pushed on by the edge of the glass, project in the inside of the cap, it should when nearly cold, be cut off by a knife and removed, so that no loose fragment may remain in the retort or tube."

(2) Varley's. Take whiting, dry it thoroughly at a red heat, and reduce it to very fine powder. Melt together 16 parts of black resin, and 1 of beeswax, and stir into the melted mass 16 parts of the dry and warm whiting, which should not be so hot as to affect the resin. (3) Singer's Electrical. Resin, 20 parts; beeswax, 4; red ochre, 4; plaster-of-Paris, 1. Dry the powders thoroughly, and add them while warm to the melted resin and wax. (4) A cheaper cement, for lining voltaic troughs, is made of 6 lb. resin, 1 lb. red ochre, 1/2 lb. plaster-of-Paris, and 1/4 lb. linseed oil. The ochre and plaster should be thoroughly dried and heated, and added to the other ingredients in their melted state. (5) Temperatures from 212° to 300° F. For cementing glass tubes, necks of balloons, etc, into metal mountings, where the apparatus is to be exposed to heat, a mixture of equal parts of red and white lead is preferable to white lead alone. If possible, the surface of the glass should be roughened, and a little tow wrapped round the part where the cement is to be applied. This cement takes some time to acquire its full degree of hardness.

In a week it will stand boiling water; in a month it will resist steam at 300° F. (149° C).

(6) Equal weights red lead and white lead; preferable to white lead alone, and may be depended on for any temperature up to 212° F. (100° C). (7) A good cement for connecting the parts of electrical or chemical apparatus may be made by mixing 5 lb. rosin, 1 lb. wax, 1 lb. red ochre, and 2 oz. plaster-of-Paris, and melting the whole with moderate heat. (8) 7 lb. black rosin, 1 lb. red ochre, 1/2 lb. plaster-of-Paris, well dried, and added while warm; heat the mass to a little above 212° F. (100° C.) and agitate it together, till all frothing ceases and the liquid runs smooth; the vessel is then removed from the fire, and the contents are stirred till sufficiently cool for use. (9) 4 oz. linseed oil added to the ingredients of (8).