142 MINUTE STRUCTURE OF THE STEM. 



403. It may be here mentioned that man}' woods have charac- 

 teristic odors ; for instance, sandal-wood, violet-wood, and many 

 of the coniferous woods, 



404. The presence of resinous matters in wood, particularly 

 when these are evenly although sparingly distributed through the 

 mass, exerts a marked effect in retarding decay. The durability 

 of the wood of Southern Cypress, even when exposed to the joint 

 action of the warmth and moisture of a greenhouse, is usually 

 attributed to their presence. But there are some cases of great 

 resistance to the influences producing decay, which cannot be 

 referred to the same mode of protection ; for instance, those of 

 Robinia Pseudacacia (or common u Locust ") and Catalpa. 



405. Various processes have been tried for destroying the 

 putrescible matters in cells, or so modifying the character of 

 the cell-wall that the wood can be protected against decay. 



406. The oldest known method of preserving wood is car- 

 bonizing, or charring, by which those constituents of the wood 

 specially liable to decay are so changed as to be no longer liable 

 to putrefaction. The wood-preserving processes known as Bur- 

 nettizing and Kyanizing have for their object the coagulation of 

 protein matters in wood-cells, thus retarding if not preventing 

 putrefaction. 



407. In Ivyanizing, a solution of mercuric chloride is forced 

 into the texture of the wood ; but the cost of this substance 

 is so great, that it has led to a general abandonment of the 

 process. 



408. In Burnettizing, the wood is impregnated with a solution 

 of zinc chloride containing about fifty-five per cent of the dry 

 chloride. This is forced into the wood under pressure. 



409. Another process - - creosoting - - depends upon the intro- 

 duction into the wood of a solution of impure creosote, a pressure 

 of about one hundred and fifty pounds to the square inch being 

 maintained until the wood has absorbed a sufficient amount of 

 the antiseptic liquid. Some of the antiseptic matters obtained 

 by a rough distillation of coal-tar are also used for preserving- 

 wood. 



It is an interesting fact that even wood which in the air 

 is specially liable to decay can be preserved for a long time if 

 deeply submerged in water. 



410. There is an appreciable difference, especially in length, 

 between the wood-cells of the earlier annual rings and those 

 which succeed them ; and Sanio has shown that an increase of 

 length of the cells occurs up to a certain period of growth, when 



