324 STRUCTURE OF WOOD, &C. 



seer-woods give nearly the same chemical results when treated 

 in the same manner. Hence, without doubt, we have good 

 reason to suspect, that the ligneous substance of all woods is 

 identical. But without stopping to discuss this question at pre- 

 sent, 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 

 Seer-zvoods. 



Sap, &c. in Greeve and Malpighi discovered in plants certain vessels, 



woods" which they suspected to be destined for the reception of air ; 



and many physiologists have supposed, that the air found shut 

 up in the vessels of plants, which (if it be really confined 

 there) would necessarily cause a reaction upon the neighbour- 

 ing vessels, with an elastic force, as variable as the temperature 

 to which this elastic fluid is exposed, and might probably con- 

 tribute to the circulation of the sap. 



It would, doubtless, be an interesting question to determine 

 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 com- 

 bining them with other simultaneous phenomena, we might 

 hope to make some discovery which may assist us a little to 

 elucidate the profound obscurity that at present conceals this 

 part of the vegetable economy. 



The specific gravity of the solid parts of a plant being known, 

 it becomes very easy to determine, in every case, the quantity 

 of air contained in its vessels and pores. 



Thefollowing example will render this position perfectly clear : 



An oak, in complete health, in a growing sta*e, was cut down 

 on the 6th of September, 1812. A cylindrical piece, six 

 inches long, and rather more than an inch in diameter, taken 

 from the middle of the trunk of this young tree, three feet 

 above the earth, weighed, when full of sap, 18T57 grammes. 



Upon plunging this piece of wood into a cylindric vessel about 

 \\ inch in diameter, and Q\ inches in height, filled with water 

 at the temperature of 62° F. it displaced 188*57 grammes of 



the 



