BOTANY. 203 



Charcoal. 



In general, the quantity of charcoal afforded by 

 woods offers a tolerably accurate indication of their 

 durability. Those most abundant in charcoal and 

 earthy matter are the most permanent, while those 

 that contain the largest proportion of the gazeous 

 elements are the most perishable. Among our own 

 trees, the chestnut and the oak are the most perma- 

 nent, and the chestnut affords rather more^ carbona- 

 ceous matter than the oak. 



The Leaves. 



Absorption and evaporation principally take place 

 from the lower surface of the leaf. 



The leaves of several plants are covered with bris- 

 tles, which in the nettle are perforated, and contain a 

 venomous fluid. 



Chemical Botany. 



M. Gay Lussac, and other French chemists, have 

 deduced three propositions, which they have called 

 laws, from their experiments on vegetable substances : 



1. That a vegetable substance is always acid, 

 whenever the oxygen it contains is, to the hydrogen, 

 in a greater proportion than in water. 



2. That a vegetable substance is always oily, or 

 resinous, or spirituous, whenever it contains oxygen 

 in a smaller proportion to hydrogen than in water. 



3. That a vegetable substance is neither acid nor 

 resinous, but is either saccharine or mucilaginous, or 

 analogous to woody fibre or starch, whenever the 



.oxygen and hydrogen it contains are in the same 

 proportion as in water. 



Sunshine. 



In the sunshine, vegetables decompose the car- 

 bonic acid gas of the atmosphere, the carbon of 



