WATER. 



WATER. 



i*inovd both from the earth and their sur- 



mosphere. 



rtholU't was of opinion that the leaves 

 of plants have the power of decomposing ^wa- 

 ter when exposed to the light of the sun. The 

 n gas, according to this distinguished 

 . which is always emitted under 

 nstances, is derived partly from the 

 ...sition of the water. "Indeed," adds 

 n, If we consider the great quan- 

 'iVilr.nr'Mi contained in plants, it is diffi- 

 how they should obtain it, 

 -.1 ihf water they absorb does not con- 

 M furnish it." (System of Chem. vol. iv. 

 These views open a field for future 

 nnt | h -tin? researches, which will 



probably lead to the establishing of new facts 

 important to the cultivator. And as 

 he chief of chemists, well said, "We 

 can only reason from facts. We cannot imi- 

 tate the powers of composition belonging to 

 regulable structures, but at least we can under- 

 stand them ; and as far as our researches have 

 rone, it appears that in vegetation compound 

 forms are uniformly produced from simpler 

 md the elements in the soil, the atmo- 

 sphere, and the earth, absorbed and made parts 

 of beaiyiful and diversified structures." (Lee- 

 fur", p. 314.) 



Pure water, therefore, is certainly not capa- 

 ble of entirely supporting vegetation. Yet, 

 ilthough it cannot produce effects so extensive 

 -?, yet its uses are many and important, 

 and it is more than probable that it is decom- 

 posed by plants, its oxygen partially evolved, 

 assimilated with carbon and 

 oxygen into a variety of vegetable substances, 

 most of which contain hydrogen in some form 

 or other : thus 

 Sugar ft composed of 



Hydrogen 



Parts. 



6-18 

 49-38 

 44-44 



100 



Gum, of 



H . 'Irogen 

 Oxygen - 

 Carbon - 



Starch, of 



rogen 

 Oxygen - 

 Carbon - 



C22 

 49-78 



100 



It would be difficult indeed to account for the 

 large proportion of hydrogen present in vege- 

 ices, without we allow that in some 

 nter is decomposed by the plant. 

 "All the hydrogen," says Professor Liebig, 

 rathrr too sweepingly, "necessary for the for- 

 mation of an organic compound is supplied to 

 a plant by the decomposition of water. (Organ. 



That plants have a strong attraction for wa- 



r from a variety of circumstances; 



thu< by their leaves and roots they separate 



the aqueous vapour of the atmosphere from 



the gases in which it is contained, and that too 



1106 



in all ordinary temperatures. This unvaried 

 presence of aqueous vapour in the atmo- 

 sphere is not less remarkable by the immense 

 importance it is to vegetation ; for without the 

 assistance which the farmer's crops derive 

 from it in dry weather, the warmth of the sun 

 would too often in the summer months wither 

 and destroy them. This beautiful arrange- 

 ment of creative wisdom did not escape the 

 attention of Davy, who noted too the variations 

 in its quantity according to the changing de- 

 mands of vegetation. The quantity of water, 

 he remarked (Elements of dgr. Chem. p. 207), 

 which exists in air as vapour, varies with the 

 temperature. In proportion as the weather is 

 hotter the quantity is greater. At 50 of Fah- 

 renheit's thermometer, air Contains about one- 

 fiftieth of its volume of vapour; and as the 

 specific gravity of vapour is to that of air nearly 

 as 10 to 15, this is about one seventy-fifth of its 

 weight. At 100, supposing that it has a free 

 communication with water, it contains about 

 one-fourteenth part in volume, or one twenty- 

 first in weight. It is the condensation of va- 

 pour by the diminution in the temperature of 

 the atmosphere which is probably the princi- 

 pal cause of the formation of clouds, and of 

 the deposition of dew, mist, snow, or hail. The 

 leaves of living plants appear to act upon the 

 vapour likewise in its elastic form, and to ab- 

 sorb it. Some vegetables increase in weight 

 from this cause when suspended in the atmo- 

 sphere, and unconnected with the soil; such 

 are the house-leek, and different species of the 

 aloe. In very intense heats, adds Davy, and 

 when the soil is dry, the life of plants seems to 

 be preserved by the absorbent power of their 

 leaves ; and it is a beautiful circumstance in. 

 the economy of nature, that aqueous vapour is 

 most abundant in the atmosphere when it is 

 most needed for the purposes of life, and that 

 when other sources of its supply are cut off", 

 this is most copious. 



And, again, when water is combined with 

 saline substances, the roots of plants separate 

 it from them in a very remarkable manner. 

 Some curious experiments of this kind were 

 made by M. Saussure. ' See SALTS, their Uses to 

 Vegetation. 



That plants have the power, when nourished 

 only with pure water, of decomposing the car- 

 bonic acid gas of the atmosphere, has been 

 shown by some very careful experiments of 

 M. Saussure. He found that some sprigs of 

 peppermint, when supplied with pure water 

 only, and allowed to vegetate for some time in 

 the light, nearly doubled the portion of carbon 

 which they originally contained. The quan- 

 tity of water which, under ordinary circum- 

 stances, plants absorb, is very considerable ; 

 thus, Dr. Hales ascertained that a cabbage 

 transmits into the atmosphere, by insensible 

 vapour, about half its weight of water daily; 

 and that a sunflower, 3 feet in height, trans- 

 pired in the same period nearly 2 Ibs. weight. 

 (Veg. Statics, vol. i. p. 5, 15.) Dr. Woodward 

 found that a sprig of mint, weighing 27 grains, 

 in 77 days emitted 2543 grains of water. A 

 sprig of spearmint, weighing 27 grains, emitted 

 in the same time 2558 grains. A sprig of com- 

 mon nightshade, weighing 49 grains, evolved 



