350 MANUAL OF TREE DISEASES 



the best all-round dressings. It should be remembered that 

 coal-tar contains a large percentage of the more toxic substances 

 (phenols, and the like) than does oil-tar, which contains no 

 phenols. The refined tars (pitch) are in all cases less toxic and 

 less fluid. The tars must be chosen with regard to their physi- 

 cal as well as chemical properties, since they must be durable 

 in hot, as well as in cold weather. 



Another pitch-like compound, asphaltum, is highly recom- 

 mended. This is the residuum of the distillation of western 

 petroleum. Its toxicity has been little investigated but it ap- 

 pears to be a desirable wound dressing. It is durable and the 

 distillations running to high temperatures (200°— 285° Fahr.) 

 leaves an asphaltum which remains solid at the temperatures 

 to which it will be exposed as a wound dressing. It must be 

 applied heated to the melted condition, or dissolved in ben- 

 zene or other liquid petroleum products (varnolene is recom- 

 mended). Many "tree paints" are solutions of asphaltum. 

 The solvent used in some of these paints often kills the living 

 tissues around the wound. Pure white lead paint makes a 

 good dressing which prevents checking to some extent. It 

 must be renewed frequently. 



Slater's cement, although not much used, is said to make a 

 suitable dressing. The wound should first be painted with a 

 disinfectant since this cement has no fungicidal properties. The 

 cement remains plastic and does not crack. Before recommend- 

 ing, it should be determined whether or not it actually prevents 

 infection. 



The capping of pruning wounds and the covering of exposed 

 surfaces with sheet metal is sometimes advised. There are 

 many reasons why such a practice is unscientific. In nailing 

 on the sheet metal, many small cracks are made in the wood. 

 Those which extend back under the cover cannot be reached by 

 a fresh coat of any dressing. The cap cannot form a permanent 

 or absolute covering and moisture will in time accumulate be- 



