IV.] VARIOUS THEORIES, &c. 



93 



4. The weight of the organic wood-substance (+ traces of 

 ash) per volume (fresh) 



Dry weight 

 Volume (fresh) 



5. The amount of water in the volume (fresh) 



Weight (fresh) dry weight 

 Volume (fresh) 



6. The amount of water in 100 units of the fresh weight 



Weight (fresh) - weight (dry) 

 Weight (fresh) 



Sachs had already shown in his paper on the 

 Porosity of Wood, that the specific gravity of the 

 solid wood-substance is 1*56, a number that was ob- 

 tained by heating small pieces of wood in solutions of 

 neutral salts to drive out the air, and then finding 

 the specific gravity of the solution in which the wood 

 just begins to sink. Hartig repeated these experi- 

 ments with birch, beech, oak and pine, both heart- 

 wood and sapwood, and found that they all remained 

 for days floating quietly at any level in a solution of 

 calcium nitrate of specific gravity i'555, whence we 

 may regard Sachs' number as confirmed. With this 

 further datum, we get 



7. The percentage volume occupied by the dry wood-wall 

 per volume (fresh) 



Weight of organic substance per 100 volumes (fresh) 

 Sp. gr. of wood substance (i.e. 1*56) 



