SEASONING AND IMPEEGNATIOX OF TIMBER 287 
extra for artificially treating them, or use more costly but 
harder and more durable timber which will not need 
artificial treatment, or again, in cases where a long life is 
not required, use the softer timber in its natural state. ^ 
Fireproof Timber. Many experiments have been made in 
the direction of rendering the timber fireproof, but not 
much practical result has been attained, and so called fire- 
proof timber has been but little used. Practical experience 
has narrowed the efficacious compounds to be used in the 
fireproof treatment of timber to ammonium chloride, 
ammonium phosphate, ammonium sulphate, calcium 
chloride, alum, borax, boracic acid, and a few others. 
The solutions, which are said to penetrate right into the 
interior of the wood, do not afi'ect the workmen's tools, and 
the treated timber will take paint, stains, varnish, etc., just 
as well as untreated wood. 
According to Professor Vivian B. Lewis, they crystallise 
in the cells of the wood, and under the influence of heat 
" the ammonium phosphate decomposes to ammonia gas 
and phosphoric acid, the former driving all air out of the 
cells and replacing it by a non-inflammable gas, whilst the 
phosphoric and boracic acids fuse and coat the cell walls 
with a glaze which, while allowing the gases from the 
decomposing cellulose to escape, prevents the access of 
oxygen from the air to carry on further combustion." 
' In 1901 there were only fifteen timber treating plants in the 
United States; in the following six j'-ears this number was increased to 
fifty. 
