628 



FARMERS' KEGISTEK 



[No. 11 



economy of nature, that aqueous vapor is most 

 abundant in the atmosphere when it is most needed 

 lor the purposes of life: and that when other sources 

 of its supply are cut off, this is most copious. 



The compound nature of water has been refer- 

 red to. It may be proper to mention the experi- 

 mental proofs of its decomposition into, and com- 

 position Irom, oxygen and hydrorren. 



If the metal called potassium be exposed in a 

 glass tube to a small quantity of water, it will act 

 upon it with great violence; elastic fluid will be 

 disengaged, which will be found to be hydrogen ; 

 and the same eti'ects will be produced upon the 

 potassium, as if it had absorbed a small quantity 

 of o.xygen ; and the hydrogen disengaged, and 

 the oxygen added to the potassium, are in weight 

 as 2 to 15 ; and if two in volume of hydrogen, 

 and one in volume of oxygen, which have the 

 weights of 2 and 15, be introduced into a close 

 vessel, and an electrical spark passed through 

 them, they will inflame and condense into 17 parts 

 of pure water. 



It is evident from the statements given in the 

 third lecture, that water forms by far the greatest 

 part of the sap of plants ; and that this substance, 

 or its elements, enters largely into the constitution 

 of their organs and solid productions. 



Water 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 conductors 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 ofwhich in northern winters 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 congelation, at which lime its volume increases 

 one-twellih, 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 per- 

 meable to the influence of the air. 



If a solution of lime in 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 be- 

 come 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 

 off carbonic acid gas, and will become quick-lime, 

 which, added to the same water, will again bring 

 it to the state of lime-water. 



The quantity of carbonic acid gas in the atmo- 

 sphere is very small. It is not easy to determine 

 it with precision, and it must difler in diflerent si- 

 tuations ; but where there is a free circulation of 

 air, it is probably never more than one-five hun- 

 dredth, nor less than one-eight hundredth 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 that it would be most 

 abundant in the lower regions of the atmosphere ; 

 but unless it has been immediately produced at 

 the surface of the earth in some chemical process, 

 this does not seem to be the case : elastic fluids of 



different specific gravities have a tendency to equa- 

 ble mixture by a species of attraction, and the 

 different parts of the atmosphere are constant- 

 ly agitated and blended together by winds or 

 othercauses. Dc Saiissure foimd lime-water pre- 

 cipitated on Mount Blanc, the highest point oi' 

 land in Europe ; and carbonic acid gas has been 

 always found, apparently indue proportion, in the 

 air brousht down from great heights in the atmo- 

 sphere by aerostatic adventurers. 



The experimental proofs of the composition of 

 carbonic acid gas are very simple. If 13 grains 

 of well burnt charcoal be inflamed by a burning- 

 glass in 100 cubical inches of oxygen gas, the 

 charcoal will entirely disappear; and, provided 

 the experiment be correctly made, all the oxygen, 

 except a few cubical inches, will be found convert- 

 ed into carbonic acid ; and, what is very remarka- 

 ble, the volume of the gas is not changed. On 

 this last circumstance it is easy to found a correct 

 estimation of the quantify of pure charcoal and 

 oxvsen in carbonic acid gas : the weight of 100 

 cubical inches of carbonic acid gas is to that of 

 100 cubical inches of oxvgen gas, as 47 to 34 : so 

 that 47 parts in weight of carbonic acid gas must 

 be composed of 34 parts of oxvgen and 13 of char- 

 coal, which correspond with the numbers given in 

 the second lecture. 



Carbonic acid is easilv decomposed by heating 

 potassium in it ; the metal combines with the 

 oxygen, and the charcoal is deposited in the form 

 of a black powder. 



The principal consumption of the carbonic acid 

 in the atmosphere, seems to be in affording nou- 

 rishment to p'ants ; and some of them appear to 

 be supplied with carbon chiefly from this source. 



Carbonic acid gas is fiirmed during fermenta- 

 tion, combustion, putrefiiction. respiration, and a 

 number of operations taking place upon the sur- 

 face of the earth ; and there is no other process 

 known in nature by which it can be destroyed but 

 by vegetation. 



After a given portion of air has been deprived 

 of aqueous vapor and carbonic acid gas, it ap- 

 pears little altered in its properties ; it supports 

 combustion and animal life. There are many 

 modes of separating its principal constituents, 

 oxygen, and azote, from each other. A simple 

 one is by burning phosphorus in a confined vo- 

 lume of air: this absorbs the oxygen and leaves 

 the azote; and 100 parts in volume of air, in 

 which phosphorus has been burnt, yield 79 parts 

 of azote ; and by mixing this azote with 21 parts 

 of fresh oxygen gas, artificially procured, a sub- 

 stance having the original characters of air is 

 produced. To procure pure oxygen from air, 

 quicksilver may be kept heated in it, at about 600^, 

 till it becomes a red powder : this powder, when 

 ignited, will be restored to the state of quicksilver 

 by giving off oxygen. 



Oxygen is necessary to some Junctions of vege- 

 tables, but its great importance in nature is in its 

 relation to the economy of animals. It is abso- 

 lutely necessary to their life. Atmospheric air 

 taken into the lungs of animals, or passed in so- 

 lution in wafer through the gills of fishes, loses 

 oxygen ; and for the oxygen lost, about an equal 

 volume of carbonic acid appears. 



The effects of azote in vegetation are not dis- 

 tinctly known. As it is found in some of the 

 products of vegetation, it may be absorbed by 



