Apart from glass and porcelain, insulators can be made from non-conducting compounds, the best of which is molded mica. Ground mica or mica dust is mixed with thick shellac until it is in a putty-like state. It may then be forced into oiled molds of any desired shape. Hydraulic pressure is employed for almost every form of molded mica that is made for commercial purposes; but as a boy cannot employ that means to get the best results, he must use all the pressure that his hands and a flat board will give. Another compound is made from pulverized asbestos and shellac, with a small portion of ground or pulverized mica added, in the proportion of asbestos, six parts; mica, four parts. Shellac is added to make a pasty mass, which is kneaded into a thick putty and forced into oiled molds until it sets. It is then removed and allowed to dry in the open air, and the mold used for more insulators. Non-conductors When working in different materials that seem adapted to electrical apparatus, it is necessary to know whether they can be used safely or not. Often a material seems to be just the thing, but if it should be a partial conductor, when a non-conductor is desired, it would be hazardous to use it. A list of non-conductors is therefore valuable to the amateur. Some of the principal non-conductors, among the many, are as follows: glass, porcelain, slate, marble, hard stone, soapstone, concrete (dry), hard rubber, soft rubber, composition fibre, mica, asbestos, pitch, tar, shellac, cotton, silk; cotton, silk and woollen fabrics, transite (dry), electrobestus (dry), duranoid; celluloid, dry wood (well seasoned), paper, pith, leather, and oil. While this account of formulæ and material might be extended, this is not necessary inasmuch as the formulæ and practical directions which have been given will answer all usual practical requirements. Insulating Varnish There are several good insulating varnishes that can be used in electrical work, the most valuable being shellac dissolved in alcohol and applied with a brush. To make good shellac, purchase the orange-colored flake shellac by the pound from a paint-store, place some of it in a wide-necked bottle, and cover it with alcohol; then cork the bottle and let it stand for a few hours. Shake the bottle occasionally until the shellac is thoroughly dissolved. It can be thinned by adding alcohol. Always keep the bottle corked, taking out only what is necessary from time to time. Another varnish can be made by dissolving red sealing-wax in alcohol and adding a small portion of shellac. This can be applied with a soft brush, and is a good varnish. When colors are to be applied to distinguish the poles, red is used for the positive current-poles and blue or black for the negative, if they are colored at all. A very good black varnish is made by adding lampblack to shellac; another consists of thick asphaltum or asphaltum varnish. This is waterproof, and dries hard, yet with an elastic finish.