The fabric is first to be varnished, to fill up the pores and render it gas-tight. For this purpose a thin linseed-oil varnish has been commonly used. To 100 parts of pure linseed-oil are added 4 parts of litharge and 1 part of umber, and the mixture is heated to about 350° Fahr., for six or seven hours, and stirred constantly. After standing a few days the clear part is drawn off for use. For the thicker varnish used on later coats, the heat should be raised to 450° and kept at about that temperature until it becomes thick. This operation is attended with much danger of the oil taking fire, and should be done only by an experienced varnish-maker. The only advantages of the linseed-oil varnish are its ease of application, and its cheapness. Its drawbacks are stickiness--requiring continual examination of the balloon envelope, especially when the deflated bag is stored away--its liability to spontaneous combustion, particularly when the varnish is new, and its very slow drying qualities, requiring a long wait between the coats. Another varnish made by dissolving rubber in benzine, has been largely used. It requires vulcanizing after application. It is never sticky, and is always soft and pliable. However, the rubber is liable to decomposition from the action of the violet ray of light, and a balloon so varnished requires the protection of an outer yellow covering--either of paint, or an additional yellow fabric. Unfortunately, a single sheet of rubberized material is not gas-tight, and it is necessary to make the envelope of two, or even three, layers of the fabric, thus adding much to the weight. The great gas-bags of the Zeppelin airships are varnished with “Pegamoid,” a patent preparation the constituents of which are not known. Its use by Count Zeppelin is the highest recommendation possible. The weight of the varnish adds largely to the weight of the envelope. French pongee silk after receiving its five coats of linseed-oil varnish, weighs 8 ounces per square yard. A double bag of percale with a layer of vulcanized rubber between, and a coating of rubber on the inside, and painted yellow on the outside, will weigh 11 ounces per square yard. Pegamoid material, which comes ready prepared, weighs but about 4 ounces per square yard, but is much more costly.