Inquiries relative to 'the Structure of Wood. 441 



The specific weight of the solid matter which com- 

 poses the fabric of these woods is so nearly alike in 

 them all, that the small variations to be observed in the 

 different experiments may perhaps be accounted for 

 otherwise than by supposing the ligneous substance to 

 be essentially different in the several species. 



The charcoal obtained from the various kinds of 

 wood, if carefully prepared, has no sensible difference; 

 and all the seerwoods give nearly the same chemical 

 results when treated in the same manner. Hence, with- 

 out doubt, we have good reason to suspect that the 

 ligneous substance of all woods is identical. But with- 

 out stopping to discuss this question at present, I shall 

 endeavour to elucidate another, no less interesting, and 

 which yields results more satisfactory. 



SECTION II. Of the Quantities of Sap and Air discovered 

 in Trees and in Seerwoods. 



Grew and Malpighi discovered in plants certain ves- 

 sels which they suspected to be destined for the recep- 

 tion of air; and many physiologists have supposed that 

 the air found shut up in the vessels of plants (if it 

 be really confined there) would necessarily cause a re- 

 action upon the neighbouring vessels, with an elastic 

 force as variable as the temperature to which this elastic 

 fluid is exposed, and might probably contribute to the 

 circulation of the sap. 



It would, doubtless, be an interesting question to de- 

 termine precisely the quantity of air contained in plants 

 in different seasons and under various circumstances. 

 By examining the variations to which this quantity of 

 air is subjected, and combining them with other simul- 



