56 



TIMBER. 



conditions, or else in the use of poisonous salts, which prevent the prop- 

 agation of fungi. 



It is also apparent, therefore, why wood decays faster in Alabama 

 than in Wisconsin, faster in the swamps than on the plains, and why 

 the presence of large quantities of decaying wood about the yard, 

 constantly producing fresh supplies of spores, stimulates decay. Cov- 

 ering with tar or impregnating 

 with creosote, salts of mercury, 

 copper, etc., enables even sap- 

 wood to last under the most try- 

 ing conditions. Contact with the 

 ground assures most favorable 

 moisture conditions for fungus 

 growth, and the higher tempera- 

 tures near the surface of the 

 ground, together with the ever- 

 present supply of spores, cause 

 rot in a post to start at the sur- 

 face more readily than 30 inches 

 below. 



The use of means to prevent 

 decay is therefore desirable where 

 timber is placed in positions favor- 

 able to fungus growth, as in rail- 

 way ties ; and all joists and timber 

 in contact with damp brick walls, 

 as also all building material whose 

 perfect seasoning is prevented by 

 the absence of proper circulation 

 of air, should be specially pro- 

 tected. In the former cases it is 

 economy to apply preservative 

 processes ; in the latter a sanitary 

 necessity. Wood covered with 

 paint, etc., before it is perfectly 

 seasoned, falls a prey to "dry 

 rot 5" the fungus finds abundance 

 of moisture, and the protection 

 intended for the wood protects its 

 enemy, the fungus. Since char- 

 coal resists the solvents of fungi, charring the outer parts of posts 

 makes, if well done, namely, so as not to open checks into the interior 

 of the wood, a very fine protection. 



Under ordinary circumstances, only the second great factor of decay, 

 i. e., the moisture condition, can be controlled. 



Fig. 36. — Cells of maple wood attacked by fungus 

 threads (Nectria cinnabarina Mayer). Section 

 of three wood fibers showing the threads of the 

 fungus branching in their cavities and consum- 

 ing the starch stored in these cells, a, interior 

 or cavity of cells; b, threads of the fuugus; c, 

 partly destroyed starch grains; d, dead portions 

 of the fungus thread together with debris; e, 

 holes bored by the fungus through the cell walls ; 

 S, starch grains just being attacked. 



