HOW WOOD IS PRESERVED 151 
cations are hard to obtain in a single substance. At the 
present time zine chloride gives good results where tim- 
bers are not subjected to continuous moisture, but in 
spite of its higher cost creosote is being more widely 
used on account of its lasting qualities. 
Small round timbers or shingles may be made quite 
durable by an application of hot creosote or coal tar ap- 
plied with a brush. This preservative kills any bacteria 
or fungi present in the outer layers and the antiseptic 
coating also renders the timber impermeable to water 
and air. Such a treatment is very beneficial but its 
effect is not lasting. Deeper penetration may be ob- 
tained by submerging small timbers to be preserved in 
a tank half full of hot preservative. This kind of 
treatment is spoken of as “open tank” preservation and 
is often done on a small scale by farmers. By heating 
creosote in a tank or open barrel to a high temperature 
and then putting the fence posts to be preserved in 
the boiling liquid and allowing them to remain until 
it is cool, a large amount of the preservative is absorbed 
and the material so treated proves very durable. Where 
deeper penetration is needed or the preservation of 
ties or timber is done on a large scale, as in the case 
of railroads or regular treating plants, the preservative 
is generally applied in a cylinder where pressure can 
be applied to force it deeply into the wood. When 
timbers are to be placed in a moist situation or where 
for any reason conditions will be favorable to rapid 
decay, complete impregnation should be obtained. This 
is possible only under high pressure and is a very expen- 
Sive operation owing to the difficulty of the process 
and the amount of preservative consumed. Complete 
impregnation is rarely used, however, and as a rule if 
the outside of timber is soaked with creosote to the 
