SCIENCE OF AGRICULTURE. P.;rtII. 



239 1. '•''•!. r is absolutely necessary t« the economy of vegetation in its clastic and fluid 

 states; and it is not d< void of use even in its solid form. Snow ami ice are bad con- 

 ductors of heal ; and when the ground is covered with snow, or the surface of the soil or 

 of water is frozen, the roots or bulbs of the plants beneath arc protected by the congealed 

 water from the influence of the atmosphere, the temperature of which, in northern win- 

 ters, is usually very much below the freezing point; and this water becomes the iirst 

 nourishment of the plant in early spring. The expansion of water during its con- 

 gelation, at which time its volume increases one twelfth, and its contraction of bulk 

 during a thaw, tend to pulverise the soil, to separate its parts from each other, and 

 to make it more permeable to the influence of the air. 



Chap. IV. 



Of the Agency of the Atmosphere in Vegetation. 



2332. The arrial medium which envelopes the earth may be studied chemically and phy- 

 sically : the Iirst study respects the elements of which the atmosphere is composed ; and 

 the second their action in a state of combination, and as influenced by various causes, or 

 those phenomena which constitute the weather. 



Sect. I. Of the Elements of the Atmosphere. 



2333. Water, carbonic acid gas, oxygen, and azote, are the principal substances compos 

 ing the atmosphere; but more minute enquiries respecting their nature and agencies are 

 necessary to afford correct views of its uses in vegetation. 



2334. That water exists in the atmosphere is easily proved. If some of the salt, called 

 muriate of lime, which has been just heated red, be exposed to the air, even in the driest 

 and coldest weather, it will increase in weight, and become moist ; and in a certain time 

 will be converted into a fluid. If put into a retort and heated, it will yield pure water; 

 will gradually recover its pristine state, and, if heated red, its former weight : so that it 

 is evident that the water united to it was derived from the air. That it existed in the 

 air in an invisible and elastic form, is proved by the circumstances, that if a given 

 quantity of air be exposed to the salt, its volume and weight will diminish, provided the 

 experiment be correctly made. 



2335. The quantity of water which exists in air, as vapour, varies with the temperature. 

 In proportion as the weather is hotter, the quantity is greater. At 50° of Fahrenheit, 

 air contains about ^ 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 ± of its weight. At 100°, supposing that 

 there is a free communication with water, it contains about T ' 5 part in volume, or ^ in 

 weight It is the condensation of vapour, by diminution of the temperature of the atmo- 

 sphere, which is probably the principal cause of the formation of clouds, and of the 

 deposition of dew, mist, snow, or hail. 



233f>. T/ie power of different substances to absorb aqueous vapour from the atmosphere 

 by cohesive attraction has been already referred to. The leaves of living plants appear 

 to act upon this vapour in its elastic form, and to absorb it. Some vegetables increase 

 in weight from this cause, when suspended in the atmosphere and unconnected with the 

 soil ; such are the house-leek, and different species of the aloe. In very intense heats, 

 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. 



2337. The existence of carbonic acid gas in the atmosphere is proved by the following 

 process: if a solution of lime and water be exposed to the air, a pellicle will speedily 

 form upon it, and a solid matter will gradually fall to the bottom of the water, and in a 

 certain time the water will become tasteless ; this is owing to the combination of the lime 

 which was dissolved in the water with carbonic acid gas, which existed in the atmosphere, 

 as may be proved by collecting the film and the solid matter, and igniting them 

 strongly in a little tube of platina or iron ; they will give out carbonic acid gas, and 

 will become quicklime, which, added to the same water, will again bring it to the state 

 of lime-water. 



2338. The quantity of carbonic acid gas in the atmosphere is very small. It is not easy 

 to determine it with precision, and it must differ in different situations ; but where there is 

 a free circulation of air, it is probably never more than one 500th, nor less than one 800th, 

 of tho volume of air. Carbonic acid gas is nearly one third heavier than the other elastic 

 parts of the atmosphere in their mixed state ; hence, at first view, it might be supposed 



