238 RESIN, TAR, PITCH. GUMS. 



colours, so that they will flow freely from the painter's brush. 

 No other known fluid would answer this purpose ; for it is the 

 only one, which will mix readily with the paint (diluting its 

 thick oil, as water would dilute a syrup or gummy fluid), without 

 in the least degree affecting its essential properties, and which 

 will also dry rapidly. 



376. The very important substances known as Tar and Pitch 

 are also obtained from trees of the same kind ; and they may in 

 fact be regarded as impure turpentines, altered by the heat 

 employed to separate them. A sort of kiln is built up of billets 

 of wood ; and round the bottom of this is a channel for drawing 

 off the fluid, which runs down whilst the wood is being burned. 

 Tar may be made from trees which no longer yield turpentine, 

 and also from those which have partially decayed on the ground. 

 Pitch is tar deprived of its more volatile part ; this may be sepa- 

 rated, either by distilling off the oil, which is an inferior oil of 

 turpentine, or by burning it ; in the last process, the volatile oil, 

 being the most readily set on fire, is burned away, and the resi- 

 nous part remains. In this manner, two barrels of tar will pro- 

 duce one of pitch ; and besides the oil, an acid passes off, by the 

 distillation of tar, which much resembles that obtained during 

 the burning of wood from charcoal, and hereafter to be men- 

 tioned urder the name of the pyroligneous. 



377. Several other Resins are yielded by plants; some of 

 which, commonly termed Gums, are of service in various arts. 

 Such is Copal, which is obtained from a species of Sumach ; but 

 though the tree will readily grow in North America and in 

 England, it requires the heat of a tropical climate to perfect its 

 juice ; and most of this product comes from Africa. Copal is 

 much valued as a varnish, on account of its hardness and trans- 

 parency ; which qualities cause it to be employed for pictures, 

 fine woodwork, and other similar purposes. Mastic is another 

 resin, which is used for similar purposes, and is obtained from a 

 tree termed the Lentisk, nearly similar to that which yields the 

 Chian turpentine. Incisions are made in the trunk and branches, 

 during the hottest parts of the summer: and the liquid juice 

 which flows from them, thickens, almost immediately that it is 



