224 



THE CABINET OF NATURAL HISTORY 



as are yet higher ; the leaf-stalk and leaf, the flower-stalk 

 and flower; as well as in the herb also, and in the lofty 

 palm, in which no such plates exist? Here it will be ne- 

 cessary to introduce the agency of a new cause, to com- 

 plete the work that has been thus begun, and of a new set 

 of machinery to supply the deficiency or absence of the ma- 

 chinery that has been already invented. 



How unsatisfactory the best of these theories is, must be 

 self-evident, even to persons unacquainted with the struc- 

 ture of vegetables. Du Petit Thouars has, therefore, pro- 

 posed a new hypothesis, which to us seems by far the least 

 objectionable. He dismisses the question of the mechanical 

 action by which the motion of the sap is maintained; think- 

 ing, with much justice, that no principle of physics, with 

 which we are acquainted, is sufficient to explain it, and he 

 therefore attributes the mere motion to an inherent power, 

 with which nature has been pleased to endow vegetables. 

 But the cause of the renewal of its motion in the spring, 

 after remaining in a quiescent state for several months, he 

 ascribes to the necessity of maintaining a perfect equi- 

 librium in the system of a plant. So that, if a consumption 

 of sap is produced at any given point, the necessity of 

 making good the space so occasioned, consequently throws 

 all the particles of sap into motion, and the same effect will 

 continue to operate as long as any consumption of sap takes 

 place. The first cause of this consumption of sap he de- 

 clares to be the development of the buds, and already 

 formed young leaves, by the stimulating action of light and 

 heat, but particularly of the latter. As soon as this deve- 

 lopment occurs, an assimilation and absorption of sap is 

 occasioned, for the support of the young leaves; a vacancy 

 in the immediate vicinity of the leaves is produced, and 

 motion immcdiatelj' takes place. 



We will not occupy ourselves with an explanation of the 

 cause of the descent of the sap : gravitation will serve the 

 purpose, in the room of a more plausible conjecture. 



But, notwithstanding all the differences which exist 

 among trees, they approach each other by insensible de- 

 grees; and yet they individually retain a peculiar set of 

 characters, and a physiognomy, which botanists call habit, 

 that renders it easy to distinguish them at great distances; 

 and more easy to eyes habituated to the sight of them, by 

 practice and long familiarity, than by the aid of theory. 



Biiffoii's Nat. Hist. 



CARBONATED SPRINGS. 



Carbonic acid gas is very plentifully disengaged from 

 springs in almost all countries, but particularly near active 

 or extinct volcanos. This elastic fluid has the property of 



decomposing many of the hardest rocks with which it 

 comes in contact, particularly that numerous class in whose 

 composition felspar is an ingredient. It renders the oxide 

 of iron soluble in water, and contributes, as was before 

 stated, to the solution of calcareous matter. In volcanic 

 districts, these gaseous emanations are not confined to 

 springs, but rise up in the state of pure gas from the soil ia 

 various places. The Grotto delle Cane, near Naples, af- 

 fords an example, and prodigious quantities are now annu- 

 ally disengaged from every part of the Limagne d'Auvergne, 

 where it appears to have been developed in equal quantity 

 from time immemorial. As the acid is invisible, it is not 

 observed, except an excavation be made, wherein it imme- 

 diately accumulates so that it will extinguish a candle. 

 There are some springs in this district, where the water is 

 seen bubbling and boiling up with much noise, in conse- 

 quence of the abundant disengagement of this gas. The 

 whole vegetation is afiected, and many trees, such as the 

 walnut, flourish more luxuriantly than they would other- 

 wise do in the same soil and climate, — the leaves probably 

 absorbing carbonic acid. This gas is found in springs rising 

 through the granite near Clermont, as well as in the ter- 

 tiary limestones of the Limagne. In the environs of Pont- 

 Gibaud, not far from Clermont, a rock belonging to the 

 gneiss formation, in which lead-mines are worked, has 

 been found to be quite saturated with carbonic acid gas, 

 which is constantly disengaged. The carbonates of iron, 

 lime, and manganese are so dissolved, that the rock is ren- 

 dered soft, and the quartz alone remains unattacked. Not 

 far off is the small volcanic cone of Chaluzet, which once 

 broke up through the gneiss, and sent forth a lava stream. 



The disintegration of granite is a striking feature of large 

 districts in Auvergne, especially in the neighbourhood of 

 Clermont. This decay was called, by Dolomieu, "la ma- 

 ladie du granite;" and the rock may with propriety be said 

 to have the rot, for it crumbles to pieces in the hand. The 

 phenomenon may, without doubt, be ascribed to the conti- 

 nual disengagement of carbonic acid gas from numerous 

 fissures. In the plains of the Po, between Verona and 

 Parma, especially at Villa Franca, south of Mantua, I ob- 

 served great beds of alluvium, consisting chiefly of primary 

 pebbles percolated by spring water, charged with carbonate 

 of lime and carbonic acid in great abundance. They are, 

 for the most part, encrusted with calc-sinter; and the 

 rounded blocks of gneiss, which have all the appearance of 

 solidity, have been so disintegrated by the carbonic acid as 

 readily to fall to pieces. The Po and other rivers, in wind- 

 ing through this plain, might now remove with ease those 

 masses which, at a more remote period, the stream was 

 unable to carry farther towards the sea; and in this exam- 

 ple we may perceive how necessary it is, in reasoning on 



