alburnum upon the juices of the bark. By per- 
forating the alburnum in a young oak, and apply- 
ing an exhausting syringe to the aperture, I easily 
drew out a small quantity of sap. I could not, 
however, in the same way obtain sap from the 
bark. I was obliged to recur to the solution of its 
principles in water, by infusing a small quantity 
of fresh bark in warm water ; the liquid obtained 
in this way was highly coloured and astringent ; 
and produced an immediate precipitate in the 
alburnous sap, the taste of which was sweetish, 
and slightly astringent, and which was colourless. 
The increase of trees and plants must depend 
upon the quantity of sap which passes into their 
organs ; upon the quality of this sap ; and on its 
modification by the principles of the atmosphere. 
Water, as it is the vehicle of the nourishment of 
the plant, is the substance principally given off by 
the leaves. Dr. Hales found, that a sunflower, in 
one day of twelve hours, transpired by its leaves 
one pound fourteen ounces of water, all of which 
must have been imbibed by its roots. 
The powers which cause the ascent of the sap 
have been slightly touched upon in the Second and 
Third Lectures. The roots imbibe fluids from the 
soil by capillary attraction ; but this power alone 
is insufficient to account for the rapid elevation of 
the sap into the leaves. This is fully proved by 
the following fact detailed by Dr. Hales, vol. i. 
of the V egetable Statics, page 114.: A vine 
branch of four or five years’ old was cut through, 
and a glass tube carefully attached to it ; this tube 
was bent as a siphon, and filled with quicksilver; 
