EXTRACTS—NATURAL HISTORY. 
23 i 
visible when the temperature is below that of itself, when it comes from our bo¬ 
dies. The glass is raised on its surface to the temperature of the surrounding 
atmosphere, and therefore does not condense the steam, but were it taken out of 
a pail of ice, wiped dry, and immediately held over the river, would it not con¬ 
dense the vapour ? Though water does not boil without heat, it would be too 
much to say it does not evaporate, or in other words, that no vapour rises from it 
when the air is above the freezing point. Upon the whole question, we must al¬ 
low that evaporation is constantly going on from the vast bodies of water on the 
surface of the globe, though its condensation, on which its visibility depends, 
must always be regulated by the temperature of the atmosphere, as before ex¬ 
plained.—E. G. Ballard .—Field Nat. Mag. 
Circulation of the Fluids of Plants. —The ascending sap rises towards 
the foliaceous parts, charged essentially, setting aside the earthy matter, with 
carbonic acid, or with bodies capable of being converted into carbonic acid; it 
loses by exhalation a very considerable quantity of the water which it contained, 
and in consequence of the decomposition of the carbonic acid gas, it gives oft’ a 
large quantity of oxygen, and is reduced to simple carbon. This sap then exists 
in a proportion, as to its elements very analagous to gum, that is to say, of water 
and carbon. 
This gum, dissolved in the water of vegetation, must run by the intercellular 
passages, both in the bark and wood, when the causes which determine the rapid 
ascent of the sap during the day, cease to operate : this is what constitutes the 
descending sap. The sap which runs down along the bark, not meeting with the 
ascending current, passes without obstruction to the root. 
The sap which runs down through the woody substances meets the ascending 
sap, which, during the day, at least, is carried up with force; these two mixing 
with the water, which is exuded from the wood by the rays aud medullary sheath, 
serve to nourish and develope the cambium into a ligneous and cortical zone. 
This gum dissolved in water in its descent by the bark, is able to assist on one 
side the secretions which are forming there, and on the other to supply nutriment 
to the cambium, or the partially organized juice, which contains the rudiment 
of the new zones. The gummy matter which also descends through the albur¬ 
num, serves likewise to nourish the cambium, or the new woody zone; this is the 
gum, which, as it descends by the bark, is stopped in its course by an annular 
section, whereby it is compelled to undergo the action of the cellules for its con¬ 
version into ligneous matter for the purpose of nourishing the bark, and develop¬ 
ing in it a protruberance formed on the trunks of woody vegetables. 
Throughout the whole of its course the gummy matter can be absorbed by the 
cellules, which are not filled, and which retain their vital action; thus all the 
cellules of the alburnum, and of the liber, like so many hygroscopic bladders 
absorb the gummy water which surrounds them; each of them elaborates it by 
its own peculiar action, and can thus according to its nature, transform the gum¬ 
my matter into fecula, sugar, or lignine, with so much greater facility, as the 
whole four substances scarcely differ; and we often in our laboratories have a 
proof that they can be converted into one another. This is especially the case 
in the experiments on the conversion of lignine and starch into hydrated sugar 
by the action of sulphuric acid. The gum and the sugar appears to be, in this 
series of decompositions, the two transitory conditions, and their extreme solu¬ 
bility in water, exposes them constantly to be drawn away towards the most 
energetic organs. 
