282 Increasing the Durability of Timber, [july, 



conditions of decay, and both must be present to facilitate 

 decomposition, that is to say, wood may be completely per- 

 meated with air, and yet will remain sound if no water is 

 present, or it may be completely saturated with water in the 

 absence of air, and yet show but little sign of undergoing 

 decomposition. But if both water and air are present in 

 wood to a sufficient extent, then the conditions of decom- 

 position are at their best, and decay will proceed more or less 

 rapidly, the rate depending on the temperature. 



The reason for this is that the destruction of wood by the 

 process of rotting depends on the w 7 ork of living organisms, 

 especially fungi, and just as moisture, air, and a suitable 

 temperature are necessary factors in the growth of higher 

 plants, so are they necessary conditions in the destructive 

 growth of the lowly plants that induce decay in structural 

 timber. When, therefore, a coat of paint is laid upon wood 

 it assists in its preservation, because it excludes moisture, 

 and, to some extent, air, but it will only be effective if the 

 wood has been thoroughly dried before the paint is laid 

 on. Should the wood be wet to begin with, or imperfectly 

 seasoned, painting it may do more harm than good, because 

 the moisture will be imprisoned and prevented escaping, and 

 conditions favourable to decay are created and maintained. 



Besides oil paint, several other substances are used to 

 waterproof wood, such as coal tar, and although the latter is 

 objectionable from some points of view, it makes an excellent 

 coat, which, being more or less elastic, admits of the contrac- 

 tion and expansion which the wood undergoes under the 

 influence of changes of temperature. A coating that cracks 

 readily is quite unsuitable, because through the openings 

 thus formed water and the spores of fungi gain an entrance, 

 and decay is rapidly set up. 



At one time — and to some extent still — posts used for gates, 

 fences, and sheds were charred for one to two feet at the part 

 that would come immediately above and below the surface 

 of the ground. This part of a post has always been recog- 

 nised as the point of weakness, because there — "between wi 

 and water," as it is called — decay always begins first and 

 progresses most rapidly. The reason for this is that near 

 the ground level the wood draws water from the soil, while 



