INTRODUCTION. 17 



quantity of air thus driven in varies according to the 

 cubic capacities of the spaces in the wood, and to the 

 temperature, etc.j and these spaces differ very largely in 

 different pieces of wood, even from the same tree. 

 Consequently, here again we get values which are not 

 constant but variable. 



Now let us see how it is possible to avoid these 

 objections, and obtain the real specific gravity of the 

 wood itself Strictly considered, this can only be done 

 by anticipating some knowledge of the structure of the 

 piece of wood, and it is only fair to point out that the 

 method was devised by a botanist, who combined his 

 knowledge of the anatomy and physiology of the wood, 

 with that of physical methods. 



The piece of wood must be so cut that all the tubular 

 cavities in it are opened, and it is then boiled for several 

 hours in some dense medium, such as a solution of some 

 stable mineral salt — say calcic nitrate, or a zinc salt — 

 until all the water and air in its cavities are displaced 

 by the medium. 



It is then found that the soaked piece of wood floats 

 at any level in a salt-solution the density of which is 

 I •56, compared with pure water as unity. In other 

 words, the specific gravity of the wood itself, apart from 

 air and water imprisoned in it, is i"56, if we call that of 

 water i, and the striking result is obtained that our 

 piece of wood is itself really heavier than — more than 

 half as heavy again as — an equal volume of water. 



Obviously, then, the chief reason why wood floats on 

 water is because it is buoyed up by the air in it ; and 

 this also explains why any piece of wood becomes 

 " water-logged," and sinks eventually, if it remains long 

 enough in a river, pond, etc. It also explains why 

 some woods cannot be floated down rivers — the air 



c 



