36 RELATIO^*S OF WATER TO VEGETABLE LIFE. 



are known to minister directly to the growth or nourishment of plants. 

 Of these, water, carbonic acid, ammonia, and nitric acid, are the most 

 important ; but it will be necessary shortly to advert to a few others, of 

 the occurrence or production or action of which we may hereafter havb 

 occasion to speak. 



§ 6. Of water and its relations to vegetable life. 



Water ia a compound of oxygen and hydrogen in the proportion, as 

 already stated, of 8 of the former to 1 of the latter by weight, or of 1 

 volume of oxygen to 2 of hydrogen. 



It is more universally diffused throughout nature than any other 

 chemical compound with which we are acquainted, performs most im- 

 portant functions in reference to animal and vegetable life, and is en- 

 dowed with properties by which it is wouderfuliy adapted to the exist- 

 ing condition of things. 



We are familiar with this substance in three several states of cohe- 

 sion,-^in the solid form as ice, in the fluid as water, and in the gaseous 

 as steam. At 32° F. and at lower temperatures, it continues solid, at 

 higher temperatu|^s it melts and forms a liquid (water), which a 

 212° F. begins to boil and is converted into steam. By this change its 

 bulk is increased 1700 times, and it becomes nearly two-fifths lighter 

 than common air, [common air being 1, steam is 0-62. J It therefore 

 readily rises into and diffuses itself tlirough the atmosphere. 



I. There are only one or two circumstances in which water in ttie solid 

 form materially aflects or interferes with the labours of the agriculturist. 



1°. During the frost of a severe winter, the soil contracts and appears 

 to shrink in. But the water contained in its pores freezes and expands, 

 and the minute crystals of ice thus formed separate the particles of the 

 soil from each other. This expansion of the water in dry soils may not 

 be equal to the natural contraction of the soil itself, yet still it is suffi- 

 cient to cause a coi.siderable separation of the earthy particles through- 

 out the whole frozen mass. When a milder temperature returns, and a 

 thaw commences, the soil expands and gradually returns to its former 

 bulk ; but the outer layers thaw first, and the particles being previously 

 separated by the crystals of ice, and now loosened by the thaw, fall off 

 or crumble down, and thus the soil becomes exposed to the mellowing 

 action of the atmosphere, wliich is enabled everywhere to pervade it. 

 On heavy clay land this effect of the winter's frost not unffe(|uenlly 

 proves very beneficial.* 



2°. In the form of snow it has been often supposed to be beneficial to 

 winter wheat and other crops. That a heavy fall of snow will shelter 

 and protect the soil and crop from the destructive effects of any severe 

 cold which may follow, there can fee no doubt. It forms a ligiit porous 

 covering, by which the* escape of heat from the soil is almost entirely 

 prevented. It defends the young shoots also from those alternations of 

 temperature to which the periodical return of the sun's rays continually 



' This alternate contraction and expansion is often injurious to the practical farmer in 

 ihrotcing out his winter wheat. Some varieties are said to be more thrown out than others, 

 and this peculiarity is sometimes ascribed to the longer and stronger roots which shoot from 

 one variety than from another ; it may, however, be occasionally owing to the different na- 

 ture of the ^oilsi 11 which the trials have been made, or when, in the same soil, to the differ- 

 ent states of dryness at different times. 



