Many other recipes have been advocated at different times to make a product resembling caoutchouc out of linseed oil in combination with other substances, but all have failed to give satisfaction, save as adulterants to pure rubber. Among the best compounds in use in rubber factories at present is one made by boiling linseed oil to the consistency of thick glue. Unbleached shellac and a small quantity of lampblack is then stirred in. The mass is boiled and stirred until thoroughly mixed. It is then placed in flat vessels exposed to the air to congeal. While still warm the blocks formed in the flat vessels are passed between rollers to mix it as closely as possible. This compound was asserted by its inventor to be a perfect substitute for caoutchouc. It was also stated that it could be vulcanized. This was found to be an error, however. The compound, upon the addition of from 15 to 25 per cent. of pure rubber, may be vulcanized and used as a substitute for vulcanized rubber. Compounds of coal tar, asphalt, etc., with caoutchouc have been frequently tested, but they can only be used for very inferior goods. The need for a substitute for gutta percha is even more acute than for artificial India rubber. A compound used in its stead for many purposes is known as French gutta percha. This possesses nearly all the properties of gutta percha. It may be frequently used for the same purposes and has the advantage of not cracking when exposed to the air. Its inventors claimed that it was a perfect substitute for India rubber and gutta percha, fully as elastic and tough and not susceptible to injury from great pressure or high temperature. The composition of this ambitious substance is as follows: One part, by weight, of equal parts of wood tar oil and coal tar oil, or of the latter alone, is heated for several hours at a temperature of from 252 to 270 degrees Fahrenheit, with two parts, by weight, of hemp oil, until the mass can be drawn into threads. Then one-half part, by weight, of linseed oil, thickened by boiling, is added. To each 100 parts of the compound one-twentieth to one-tenth part of ozokerite and the same quantity of spermaceti are added. The entire mixture is then again heated to 252 degrees Fahrenheit and one-fifteenth to one-twelfth part of sulphur is added. The substance thus obtained upon cooling is worked up in a similar manner to natural India rubber. It has not been successfully used, however, without the addition of a quantity of pure rubber to give it the requisite elasticity. A substitute for gutta percha is obtained by boiling the bark of the birch tree, especially the outer part, in water over an open fire. This produces a black fluid mass, which quickly becomes solid and compact upon exposure to air.