44 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



5. Durability of Timbers. 



By Percy Groom. 



Although our practical knowledge of the comparative 

 durability of a number of familiar timbers is not inconsiderable, 

 our detailed statistical information on the subject is exceedingly 

 restricted. There is consequently not sufficient evidence avail- 

 able to permit of the formation of any reliable guiding principles 

 in regard to the durability of woods, and we are dependent for 

 any guidance upon merely empirical rules of limited application. 

 Practical men have been familiar for centuries with the fact that 

 sapwood and some light-coloured timbers are more perishable 

 than coloured heart-woods, and for a long time must have known 

 that darker samples are often more durable than lighter 

 samples of one and the same timber. Hence very early 

 in the history of the usage of wood must have come the 

 idea that coloured woods are more durable than less deeply 

 coloured or white woods, and that the deposit of colouring 

 matter is associated with the increase of durability of wood. 

 These familiar ideas have been transmuted into definite 

 propositions, for instance on page 100 of volume v. of Schlich's 

 Manual of Forestry (an English version oi Qd^ytx's Forstbenutzung). 

 Here the following statement is made : " The presence of 

 colouring matter in heart-wood, however, increases durability 

 greatly." This statement is certainly premature; for even if it 

 were universally true that more deeply coloured timber is more 

 durable, and if the increased depth of colour were due to the 

 deposit of colouring matter, the latter deposit and the increased 

 durability may be two different results of one common cause. 

 On the same page we also find written : " Mayr also states that 

 the more intense the colour of the heart-wood the more durable it is." 

 This proposition old in idea, empirical in nature, and insufficiently 

 tested by fact, is, by Professors Gayer and Mayr, styled " Mayr's 

 Law " : it is at most a hypothesis, and at present a mere rule of 

 limited application. In support of it we find, on the same page 

 of the volume cited, a list of timbers grouped according to 

 depth of colour and in correspondence therewith described as 

 "very durable," "durable," and "not durable," as the colour 

 decreases in intensity. Yet the list is satisfactory from the 

 point of view neither of gauging of colour nor of estimate of 



