DURABILITY. 93 



for forest tramwaj'S, sledge -roads and slides, is evidently as 

 subject to decay as railway-sleepers, 



(e) Places with little Circulation of Air. — These are frequently 

 very damp, and if they are also warm, as in cellars, underground 

 vaults, stables, engine-rooms, work-rooms, and crowded and 

 badly ventilated human dwellings where clothes are dried and 

 little attention paid to cleanliness, then the wooden fittings of 

 such places cannot be very durable. 



Wood used as mining-props is under similar conditions, and 

 hardly anywhere is such a quantity of wood rapidly used-up as 

 in mines, where sprucewood only lasts four to six years. 



Even in this case there is a great difference in the durability 

 of the wood under different circumstances, and where props 

 are used in dry mountain mines, and the wood comes in contact 

 with antiseptic substances such as copper and zinc, or in salt 

 mines, woodwork may be very durable. Larchwood has lasted 

 more than sixty years in salt-mines, still remaining almost 

 perfectly sound. 



Although in all the above usages of wood, infection by fungi is 

 always the cause of decay, it is damp, warm places where venti- 

 lation is ineffective which favour infection by a growth of the 

 fungi. In such places, the presence of the fungi is evidenced by 

 the masses of mycelia produced by Merulius lacrimans * (the 

 chief cause of dry-rot) and Pol>/norus vaporarius, which are 

 found on the woodwork of cellars and on beams and floors laid 

 directly on the ground without ventilating spaces below them. 



5. Classification of Woods in Order of Diirahility. 



From all that has gone before it may be conjectured that it is 

 impossible to attribute fixed periods of durability to certain 

 timbers, and that it is difficult even to classify different timbers 

 approximately according to the absolute powers of durability. 

 If, however, durability is taken as a measure of the duration of 

 a sound condition in timber under the worst possible external 

 circumstances to which it may be exposed when utilized, some 

 attention being paid to its special anatomical structure, timbers 

 may be arranged in the following groups, in order of durability:^ — 



* Vide Hartig's Diseases of Trees, oj). cit. p. 74. 



