FORESTS AND MANKIND 



Three things are necessary to these fungi in order that they 

 may destroy: food, moisture and air. If we can remove any 

 one of those necessities we can prevent decay. Thus, when we 

 thoroughly dry a piece of wood we remove the moisture and 

 fungus can not live in it. When a piece of wood is submerged, 

 air is excluded, and the wood can not decay. Paint and varnish 

 on the surface of wood prevent the entrance of decay. 



But the most effective method of fighting the decay fungi 

 is by poisoning their food supply. For this purpose wood users 

 have selected various preservatives which depend, for their ef- 

 fectiveness on poisoning the wood tissues. Usually the wood is 

 either covered or impregnated with creosote or some other sub- 

 stance poisonous to the fungi. In more thorough methods of 

 wood preservation, as in treating railroad ties, the ties are first 

 seasoned carefully, then stacked in cars and rolled into large 

 cylinders where live steam is admitted under pressure. After 

 the wood is softened by this steam a vacuum is created to suck 

 the air and moisture out of the wood cells. Next the preserva- 

 tive, usually creosote or zinc chloride, is forced into the cylinder 

 and kept under pressure until enough has been absorbed by the 

 wood to protect it. Twenty-five to thirty-five pounds of pre- 

 servatives are often forced into each tie. By this treatment the 

 life of a tie which untreated would be about seven years may 

 be increased to more than fifteen. 



But the actual saving means more than simply doubling the 

 life of a tie. It means that railroads, instead of relying solely 

 on certain durable species like white oak or cypress species 

 now getting scarce can make use of a number of other kinds 

 of wood that untreated would be valueless, because they rot 

 so quickly. 



