n.j ITS PROPERTIES AND VARIETIES 29 



deprived of its air will sink rapidly, being about half 

 as heavy again as water, volume for volume. 



The point just now, however, is not to discuss these 

 matters in detail, but rather to indicate that, other 

 things equal, the density of a piece of timber will be 

 greater, the more of that closely-packed, thick-walled 

 autumn wood it contains ; while the timber will be 

 specifically lighter and contain more air when dry, 

 the greater the proportion of the looser, thin-walled 

 spring wood in its " annual rings." In other words, 

 if we could induce the cambium to form more autumn 

 wood and less spring wood in each annual ring, we 

 could improve the quality of the timber ; and, in view 

 of the statement which has been made, to the effect 

 that large quantities of timber of poor quality reach 

 the Continental wood-yards every year, this is ob- 

 viously an important question, or at any rate may 

 become one. The remainder of this chapter must be 

 devoted to this question alone, though it should be 

 mentioned that several other questions of scientific 

 and practical importance are connected with it. 



The first point to notice is that the cambium-cells, 

 like all other living cells which grow and divide, are 

 sensitive to the action of the environment. Jf the 

 temperature is too high or too low, their activity is 

 affected and may even be brought to an end ; if the 



