52 PROPERTIES OF WOOD. 



here, as he did not consider the influence of the weather at 

 the time of his observations, nor the degree of moisture of 

 the soil, individual variations, etc. H. Mayr's observations * 

 show that the degree of wetness of the sap wood depends on 

 the relative atmospheric humidity, which varies day and night ; 

 also on the weather, for after rain the stem may become 

 gorged with water, and drier after a period of drought. Hence 

 during any month the so-called maximum volume of water 

 in a tree may be diminished ; it varies also with the nature 

 of the soil and in individual trees. 



When felled stems lie in the forest with or without bark, 

 their wetness varies according to the relative air-humidity or 

 the raininess or dryness of the weather. Evaporation, how- 

 ever, on the whole, preponderates over high relative humidity 

 or wet weather. After a stem has lain for some time in the 

 forest, its sp. weight is that of forest-dry wood ; this is 

 less constant than its green weight or air-dry weight, but is 

 always intermediate between them. In logs and firewood the 

 upper parts approach air-dry wood in weight, those parts on 

 the damp ground, that of green wood. Wood that is felled in 

 winter and brought out of the forest at the beginning of spring 

 weighs nearly as much as green wood ; only split firewood 

 shows any appreciable reduction in weight. The weight of 

 green and forest-dry wood is of practical importance in wood- 

 transport. 



Wood becomes air-dry, or seasoned, only a long time after 

 the felling, and the more rapidly the more the wood has been 

 subdivided. Balks and thick planks must be kept for years 

 and protected from rain or from resting on the ground before 

 they become air-dry. Air-dry wood still contains 10 15 

 per cent, of its weight, in water. This is held firmly by 

 adhesion in the cell-walls, and its mass fluctuates with the 

 relative atmospheric humidity ; hence in order to expel it 

 artificial heating . to 100 110 C. is required, at which 

 temperature all the water passes into steam. In this way 

 the absolutely dry weight of wood is determined. This weight 

 has a scientific value only, in comparing the weights of woods 



* JI. Mayr. " Uber den i'oi'stliclu-ii \\Vri <lcr ia\m'ii\Y:n -ti^ iiblichen Methodi-n 

 y.ur Beslinnmiiig tier Qualitlit der Hol/cr." IMS'.). 



