236 PRINCIPLES OF AMERICAN FORESTRY. 



wood, and, although not as good for this purpose as coal 

 tar, it is very desirable. As with all other preservatives 

 that are applied to the surface, the wood should be very 

 dry before it is applied, and the wash should be applied 

 evenly over all the exposed parts. It is on account of 

 the lime washing out of the mortar that the shingles 

 on a roof just below the chimney last longer than on other 

 parts of the roof. But if whitewash is to be applied to 

 shingles, it should be applied before they are laid by 

 dipping. 



Charring those parts of posts or timbers which come in 

 contact with the ground is a good preventive, provided a 

 thick layer of charcoal is formed and the work so carefully 

 done as not to cause the timber to crack, since deep crack- 

 ing exposes the interior to decay. If the work is not 

 carefully done the timber may be seriously weakened. 



Antiseptics. The impregnating of timber with sul- 

 phate of copper (blue stone), sulphate of iron (green 

 copperas), chloride of zinc, creosote, salts of mercury, or 

 other similar material, has the effect, when properly 

 done, of greatly increasing its durability. Such anti- 

 septic substances have the power of destroying the rot- 

 producing fungi. The materials are often applied to fresh 

 logs. If dry timber is to be treated, it is first boiled or 

 steamed to open the cells. A hollow cap connected w r ith 

 a force pump is placed over one end, and the liquid forced 

 through the cap into the wood, which results in forcing 

 out the sap at the opposite end and replacing it with the 

 antiseptic, but the more common method is that described 

 below as the treatment given in impregnating railroad 

 ties in Europe. All the antiseptics mentioned have been 

 used to some extent for this purpose, but for various rea- 

 sons chloride of zinc is now most generally used. . Railroad 

 ties thus treated last much longer than those not so treated. 

 Impregnation also to some extent renders wood fire-proof. 



