TREE SURGERY 349 



is most generally used and is recommended. Asphaltum, both 

 melted and dissolved, is also a desirable substance to use. 



Crude tars are obtained largely from bituminous coal, pe- 

 troleum and wood, when these substances are burned in closed 

 retorts. The temperature at which the distillation is carried 

 out determines largely the chemical character of the crude tar. 

 The products are named according to their source, coal-tar, 

 oil-tar and wood-tar. The larger part of the coal-tar is produced 

 in coke- and coal-gas manufactories. Bituminous coal is dis- 

 tilled at temperatures varying from 1500° to 3000° Fahr. until 

 the charge has been reduced to coke. The tar, resulting as a 

 by-product, varies in its nature according to the character of 

 the coal and the temperature used. The tar contains mainly 

 hydrocarbons of the aromatic series, such as phenols, naphtha- 

 lenes and anthracenes. Coal distilled at relatively lower tem- 

 peratures contains less of the aromatic hydrocarbons and a 

 greater or less quantity of paraffin hydrocarbons. Oil-tar is 

 largely produced as a by-product of water-gas production. This 

 tar is characterized by an absence of the phenols (tar-acids) and 

 a greater or less quantity of the different hydrocarbons. Wood- 

 tars are quite different and contain, in particular, less of the 

 aromatic hydrocarbons. Thus it is seen that crude tars may 

 vary greatly and while some make suitable wound dressings 

 because of their consistency and chemical nature, others do not. 



In the further distillation of crude tars, three general classes 

 of substances are obtained, — oils lighter than water, oils heavier 

 than water and pitch or refined tar. The oils heavier than water 

 are known as creosotes. Carbolineum is a trade name for a 

 coal-tar creosote distilled at a high temperature. Creosote or 

 carbolineum are not recommended for wound dressings, since 

 neither is of a satisfactory consistency for making a permanent 

 coating. In addition, they penetrate deeply and are said to 

 injure, in many cases, the living tissues around the wound. The 

 crude tars (usually coal-tar) are most genera;lly used and are 



