Book III. OF THE ATMOSPHERE. 347 



2264. IFater is absolutely necessary to the economy of vegetation in its elastic and fluid 

 state; and it is not devoid of use even in its solid form. Snow and ice are bad con- 

 ductors of heat; 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 are 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 first 

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

 tion, 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. 



2265. The aerial medium which envelopes tHe earth may be studied chemically and phy- 

 sically ; the first 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. 



2266. Watery carbonic acid gas, oxygen, and azote, are the principal substances composing 

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

 necessary to afford correct views of its uses in vegetation. 



2267. That water exists in the atmosphere is easily proved. If some of the salt, called 

 muriate of lime, that 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. And 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. 



2268. The quantity ofxmter which exists in air, as vapor, varies with the temperature. In 

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

 air contains about one 50th of its volume of vapor ; and as the specific gravity of vapor 

 is to that of air nearly as 10 to 15 ; this is about one 75th of its weight. At 100, sup- 

 posing that there is a free communication with water, it contains about one 14tli part 

 in volume, or one 21st in weight. It is the condensation of vapor by diminution of the 

 temperature of the atmosphere, which is probably the principal cause of the formation of 

 clouds, and of the deposition of dew, mist, snow, or hail. 



2269. The j^ower of different substances to absorb aqueous vapor from the atmosphere by 

 cohesive attraction has lieen already referred to (2102.) The leaves of living plants ap- 

 pear to act upon this vapor 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 vapor 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. 



2270. 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 atmo^ 

 sphere, 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 quick -lime, which, added to the same water, will again bring it to the state 

 of lime-water. 



2271. The quantity of carbonic acid gas in the atmosphere is very small. It is not easy 

 to determine it with precison, 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 the 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 



