GASES. 



GASES. 



an explanation of facts. It is probable that 

 the early Greek and Italian philosophers were 

 farther led to this knowledge of the advantages 

 of air to vegetation, from noticing the power 

 which some eastern plants possess, such as the 

 Flos ceris and others, of entirely supporting 

 themselves upon the nourishment they derive 

 from the atmosphere, even when suspended by 

 a string from the ceiling of a room, many 

 parasitical plants subsist upon hardly any 

 thing else ; thus, some of the mosses of this 

 country cling to life, and even grow well, in 

 situations where hardly any thing except air 

 and moisture can nourish them : some of the 

 aloe tribe do the same. 



Carbonic acid gas. When, however, later 

 ages had acquired the knowledge that it was 

 only a portion of the air that maintained vege- 

 table and animal life, or supported combus- 

 tion, new views opened upon the chemical 

 philosopher. It became then a question of 

 considerable interest to ascertain which por- 

 tion of the atmosphere it was that the plant ab- 

 sorbed ; and it was speedily ascertained by 

 t)r. Priestley and other chemists, that the por- 

 tion of the atmosphere which the leaves of all 

 plants absorb in the light is the carbonic acid 

 gas or fixed air a gas composed of 27-27 parts 

 carbon, and 72-73 parts oxygen, and that this 

 carbonic acid gas is always contained in the 

 atmosphere, in the proportion of about one part 

 in 500. The question thus became one of some 

 interest to ascertain, whether a larger volume 

 of carbonic acid gas would promote, in a still 

 greater degree, the growth of plants, such as in 

 an impure, confined portion of air spoiled by the 

 breathing of animals, or exhausted of its oxy- 

 gen gas or vital air by combustion, since both 

 these varieties of air contain a very consider- 

 ably increased proportion of carbonic acid gas. 

 Many very accurate experiments speedily de- 

 monstrated that such foul air materially in- 

 creased the luxuriance of vegetables confined 

 in them, and that plants possessed also the 

 power of restoring to such exhausted air the 

 portion of oxygen which either fire or the 

 breathing of animals had removed: thus, a 

 confined portion of air, in which a mouse had 

 died in ten minutes for want of air, having had 

 a sprig of mint introduced into it for some 

 hours, was then found to be so replenished 

 with vital air, that a second mouse being placed 

 in it lived as long as the former mouse ; and, 

 by similar treatments, a lighted taper being 

 merely substituted for the mouse, the same ef- 

 fect was produced the exhausted air was 

 again replenished with oxygen gas. 



These facts naturally opened new views. It 

 then became an interesting object to ascertain 

 the proportion of the carbonic acid gas in the 

 atmospheric air, which possessed the maxi- 

 mum advantage to vegetation ; and it was 

 found that, in pure carbonic acid gas, plants 

 would not vegetate at all, or in air containing 

 75 per cent, of it, but that, when the proportion 

 present in common air was reduced to 50 per 

 cent., then the plants confined in it slowly vege- 

 tated, and that they grew more freely when the 

 proportion was 25 per cent.; still better when 

 it was 12 per cent.; and that when it was re- 

 duced to only 9 per cent., then they nourished 

 66 



much better than in common atmospheric air. 

 It was remarked, however, that the increased 

 presence of carbonic acid gas was only bene- 

 ficial to plants when they were vegetating in 

 the light, but that, when this was excluded, the 

 carbonic acid gas was rather prejudicial to 

 their growth than otherwise; that, in fact, all 

 plants, though they absorb it in the light, yet in 

 the dark emit this gas. It was ascertained, 

 however, that the presence of it in their atmo- 

 sphere was absolutely essential to all plants 

 vegetating in the light; that they grew when 

 it was present, and that all vegetation was 

 stopped by its withdrawal. 



These results naturally led to the additional 

 inquiry, Whether the presence of carbonic acid 

 gas in water produced the same results on 

 plants, since it was well known that, when 

 plants were immersed in water and exposed to 

 the sun's rays, they emitted bubbles of oxygen 

 gas, by decomposing the carbonic acid gas, and 

 setting its oxygen free. Various kinds of water 

 were tried, containing different proportions of 

 carbonic acid gas ; and the beneficial result 

 upon vegetation was found to be exactly pro- 

 portionate to the quantity of carbonic acid gas 

 which they contained. In pump-water, they 

 yielded the most oxygen; from river water a 

 smaller quantity; but from boiled water little 

 or none. Now, by boiling, all the gases are 

 driven out of water, and this is the reason why 

 such water is flat and insipid. And yet it was 

 found that when the boiled water was again 

 impregnated with carbonic acid gas, those 

 plants confined in it emitted as much oxygen, 

 gas as they did before it was boiled; and, 

 finally, that when the plants had exhausted the 

 water of carbonic acid gas, then they ceased 

 to emit oxygen. 



The quantity of carbonic acid gas which is 

 emitted by plants varies in different species. 

 Thus, M. Saussure found that the purple loose- 

 strife (Lythrwn salicaria) absorbed in 12 hours 

 7 or 8 times its bulk ; while the Cactus opuntia, 

 in common with other fleshy-leaved plants, did 

 not absorb above one-fifth of that amount. In 

 these experiments, however, the atmosphere in 

 which the plants were confined contained 7jf 

 per cent, of this gas ; so that when they are 

 vegetating in the open atmosphere, in which 

 the proportion of this gas does not exceed one 

 part in 1000, the quantity absorbed is consider- 

 ably less. 



This absorption of the carbonic acid gas, the 

 cultivator should clearly understand, influences 

 in a great degree the composition of the plant. 

 All those vegetable, carbonaceous, nutritious 

 substances which are found in plants, such as 

 gum and sugar, are increased in quantity by 

 its copious supply; for when this gas is no 

 longer secreted by the plant, its health becomes 

 languid, and its compositon more watery. Thus 

 a yssus vegetating in the dark (when carbonic 

 acid gas is emitted by plants), was analyzed by 

 M. Chaptal, and found to contain only l-89th 

 of its weight of carbonaceous matter; but when, 

 after it had been allowed to vegetate for 30 days 

 in the light, it was again examined, it was found 

 to contain l-24th of its weight of carbonaceous 

 matter. Similar results were obtained by M. 

 Sennebier, who found that when plants were 

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