WATER. 



WATER, 



removed both from the earth and their sur- 

 rounding atmosphere. 



M. Berthollet was of opinion that the leaves 

 of plants have the power of decomposing wa- 

 ter when exposed to the light of the sun. The 

 oxygen gas, according to this distinguished 

 philosopher, which is always emitted under 

 these circumstances, is derived partly from the 

 decomposition of the water. "Indeed," adds 

 Dr. Thomson, "If we consider the great quan- 

 tity of hydrogen contained in plants, it is diffi- 

 cult to conceive how they should obtain it, 

 provided the water they absorb does not con- 

 tribute to furnish it." (System of Chcm. vol. iv. 

 p. 349.) These views open a field for future 

 and highly interesting researches, which will 

 probably lead to the establishing of new facts 

 highly important to the cultivator. And as 

 Davy, the chief of chemists, well said, "We 

 can only reason from facts. We cannot imi- 

 tate the powers of composition belonging to 

 vegetable structures, but at least we can under- 

 stand them ; and as far as our researches have 

 gone, it appears that in vegetation compound 

 forms are uniformly produced from simpler 

 ones ; and the elements in the soil, the atmo- 

 sphere, and the earth, absorbed and made parts 

 of beautiful and diversified structures." (Lec- 

 tures,]). 314.) 



Pure water, therefore, is certainly not capa- 

 ble of entirely supporting vegetation. Yet, 

 although it cannot produce effects so extensive 

 as these, yet its uses are many and important, 

 and it is more than probable that it is decom- 

 posed by plants, its oxygen partially evolved, 

 and its hydrogen assimilated with carbon and 

 oxygen into a variety of vegetable substances, 

 most of which contain hydrogen in some form 

 or other : thus 



Sugar is composed of 



Parts.- 



Hydrogen - 6-18 



Oxygen 4938 



Starch, of 



Hydrogen 

 Oxygen - 

 Carbon - 



100 



622 

 49-78 

 44- 



100 



It would be difficult indeed to account for the 

 large proportion of hydrogen present in vege- 

 table substances, without we allow that in some 

 instances water is decomposed by the plant. 

 "All the hydrogen," says. Professor Liebig, 

 rather too sweepingly, " necessary for the for- 

 mation of an organic compound is supplied to 

 a plant by the decomposition of water. (Organ. 

 Chern. p. 66.) 



That plants have a strong attraction for wa- 

 ter is evident from a variety of circumstances; 

 thus by their leaves and roots they separate 

 the aqueous vapour of the atmosphere from 

 the gases in which it is contained, and that too 

 1094 



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 nftmlhs wither 

 and destroy them. This beautiful arrange- 

 ment of creative wisdoni 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 



