IV.] VARIOUS THEORIES, &C 91 



as have been considered. And again, the atmo- 

 spheric pressure outside the plant has no appreciable 

 effect on the process ; it is controlled and regulated 

 by alterations in the density of the imprisoned air. 



Hartig's methods of obser\'ation are worth a short 

 description here, because they give not only insight 

 into several peculiarities of wood, but also an idea of 

 the very different points of view involved. 



In the forest, on the spot where the trees 

 were felled, the pieces of wood to be examined 

 were chosen and at once weighed, because they 

 rapidly lose weight after exposure to air. In choosing 

 them, the usual plan was to cut up the trunk into 

 blocks, and to select blocks from the various heights, 

 about 2 — 3 meters apart, splitting them up as follows : 

 two opposite wedge-shaped segments were cut out of 

 the circular block, and each separated into three parts 

 — the inner part comprising the heart-wood, the outer 

 part sap-wood, and the middle one both sap-wood 

 and heart-wood. The pieces thus obtained weighed 

 from 300 to 700 grams each, and after weighing were 

 packed and despatched to the laboratory. 



Here, the first thing to do was to determine the 

 volume in the fresh state,-^ which was done by reading 



^ Since the wood does not shrink sensibly until it has lost much water, 

 it was not necessary to do this m the forest, but it was done next day. 



