134 VETERINARY HYGIENE 



has been derived from the outer part of the tree, and is therefore 

 almost entirely composed of the newest of the sapwood, or has 

 been derived from a small and therefore immature tree. Such 

 timbers should be discarded, but for rough or unimportant work 

 they may be used if placed in an airy position, the " waney edge " 

 being placed uppermost in the case of rafters or floor joists. 



Characteristics of Good Timber. Timber of good quality 

 should be straight in the grain, free from large, or dead, or loose 

 knots, shakes and waney edges. It should be thoroughly seasoned, 

 uniform in colour, and as free from sapwood as possible. 



Timber, when cut with a saw, should give out a fresh smell, 

 should cut freely without clogging, and present a clear and firm 

 surface, free from any dull or spongy appearance. A spongy 

 appearance should lead one to suspect dead timber, that is, timber 

 cut from a tree which had died before being felled. 



In selecting timber it should be remembered that closely-set, 

 dark-coloured annual rings indicate strength, whilst wide-set annual 

 rings, and rings of a light colour, indicate the newer or sapwood. 

 Good timber, when struck, should give forth a clear, ringing 

 sound. 



Grain of Timber. A log of wood, cut into planks in a direction 

 parallel with the medullary rays, presents a pleasing figured appear- 

 ance known as " silver grain " when the timber is dressed. This 

 method of converting timber into planks is not always economical, 

 and is usually confined to the production of the better classes of 

 wood, such as oak, mahogany, maple, walnut, &c., for fine joiner- 

 work. 



It is usual to cut timber for structural work so that as many 

 cut battens, deals, planks or boards may be obtained from the log 

 as is possible. 



Dry Rot in Timber. From the date of the felling of a tree 

 timber is liable to attack by " dry rot,"* a disease which is usually 

 most active when the timber is in position in a structure. 



This disease, which is due to several fungi, especially merulius 

 lacrymans, thrives most readily in situations which are unventilated 

 and have a warm, damp atmosphere. The spores of the fungus 

 may be in the timber before it is built into position, or they may 

 be carried by wind, on joiners' tools, or by the temporary or 

 permanent proximity of old infected timber. 



* The term " wet rot " is applied to decay started in the standing tree ; " dry rot " 

 to " the form of decay induced in timber that is apparently sound when first 

 used as constructional material." See Journ. Board of Agric., Aug., 1916, 

 XXIIL, No. 5, p. 465. 



