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^HU 'MOPICAL AGRICULTURIST. [March i, my, 



Dr. Priestley, the great discoverer of oxygen gas, 

 ■afterwards made the equally important discovery 

 that the leaves of plants have the power, under the 

 influence of sunlight, of absorbing carbonic acid by 

 their under surfaces, holding the carbon in the form- 

 ation of vegetable compounds, and emitting from tbe 

 npper surfaces the oxygen gas set free by the de- 

 composition of the carbonic acid. The demonstra- 

 tion is so simple that anybody can repeat it. No 

 cumbrous and complicated apparatus is required. A 

 bell glass or candy jar and a bucket of water is all 

 the apparatus needed for the splendid demonstra- 

 tion. We have our bell glass full of water, the rim 

 dipping an inch or so beneath the surface of the 

 water lu the bucket. A blade of rapidly grow'ing 

 corn is bent over, and inserted into the bell glass. 

 At once, bubbles of gas collect on the upper surface 

 of the blade and rise through the water, displacing 

 the water and driving it down into the bucket. In 

 this way, wg may collect, in a few minutes, enough 

 of the gas for experiment. The gas is oxygen, for 

 an extinguished taper containing the slightest spark, 

 when introduced into the gas, flares up immediately. 

 Now what has happened V To make the explanation 

 perfectly clear, two preliminary explanations are 

 required. (1.) Carbonic acid is a compound of car- 

 bon and oxygen, in the proportion of one atom of 

 ca bon to tvio atoms of oxygen. Hence it is written 

 CO 2- ("2) Water, under ordinary pressure, absorbs 

 its own volume of carbonic acid, so that all water 

 contains this gas. 



Under the influence of solar light, the under sur- 

 face of the leaves have sucked in or absorbed the 

 carbonic acid present in the water, the carbon has 

 been held to form the vegetable skeleton, and the 

 oxygen evolved from the upper leaf surfaces. And 

 as carbonic acid contains its own volume of oxygen, 

 the volume of oxygen evolved from the upper leaf 

 surfaces is equal to the volume of carbonic acid 

 absorbed by the under leaf surfaces. 



We may vary the experiment as following: — Let 

 us have a trough of water furnished with a shelf. 

 On this shelf we place a pot containing a vigorous 

 geranium. Let down a bell glass over the geranium 

 pot, so that the water will be a little above the 

 mouth of the bell glass. The air is confined. Water 

 prevents tbe access of the outer air. Place the whole 

 arrangement in the sunlight. In an hour or so, the 

 air of the bell glass is found to be utterly destitute 

 of carbonic acid. 



There can be no mistake. Nothing is more easily 

 detected in the atmosphere than carbonic acid (CO^.) 

 When lime water is exposed in an atmosphere con- 

 taining carbonic acid, immediately a thin crust, re- 

 sembling' ice, begins to be formed on the surface of 

 the water. This film or crust is carbonate of lime, 

 formed by the combination of carbonic acid with lime. 

 If touched with any acid it will effervesce, the effer- 

 vescence being due to the liberation of carbonic acid. 



The leaves of plants then suck in carbonic acid, 

 decompose it, fixing the solid carbon in building 

 up its compounds and evolving the oxygen gas. 

 Now, the great pros-imate compounds of plants, as 

 starch, sugar, f^uni, woody fibre, are simply carbon 

 combined with the ekments of water. AH <.if them 

 may be easily resolved into carbon and water. We 

 arc now prepared to understand how simply they 

 arc formed m the vegetable economy. They are all 

 formed by the union of carbon, set free in the de- 

 composition of carbonic acid, with the elements of 

 water introduced from the earth by the roots of 

 f^rowins plants. 



We cou'jider then the carbonic acid of the atmo- 

 siphere the primary source of all the carbon of plants. 

 Plants sometimes i^row under circumstaiires wjiich 

 Utterly exclude any other source of supply, 



If wc plant a griiiu of wheat— the experiment has 

 been tried- in pulverized silica, and keep it properly 

 moistened with rain water, it will swell and sprout 

 and germinate. A true stalk is formed, which grows 

 enough to contain three times as much carbon as 

 the planted grain. Whence this excess of carbon? 



Evidently and necessarily the carbonic acid of the 

 atmosphere, for there was no carbon in any form 

 ju tUv Hitiijcia! soil wliicli grt^v, tbe plaut, Wc 



witness the same thing, on a larger scale, when 

 clay, containing no carbon in any form, is dug up 

 from great depths, and exposed to the atmosphere. 

 Seeds are dropped upon the clay; they germinate 

 and grow to a greater or less extent. The carbon 

 obtained by vegetation growing under such circum- 

 stances must come exclusively from the atmosphere, 

 for the soil contains none. 



Let us consider how plants grow upon a purely 

 mineral foundation. Consider an acre of granite. 

 Granite is composed of three minerals : quartz, felds- 

 par, and mica, and none of these minerals contain 

 carbon. Seeds dropped by birds or wafted by the 

 winds are deposited upon this acre of granite. Under 

 the necessary conditions of warmth, moisture, and 

 access of air they germinate, and develop into scant 

 and stinted vegetation, which contains, say, three 

 times as much carbon as the seeds whi«h produced 

 it. The atmosphere by its carbonic acid necessarily 

 furnished the whole of this excess of carbon. But 

 the vegetation of the second year will be more 

 vigorous and heavy for two reasons. First, the roots 

 have penetrated somewhat into the hard granite in 

 search of food, and to a small extent have commi- 

 nuted it, and this comminution will be still greater 

 from the freezes of the coming winter. As the roots 

 decay, water will fill up the little channels they 

 made in the rock, and in freezing will expand and 

 so split and peel off the surface of the gramte. The 

 vegetation of the first year will be mixed with this 

 reduced and disintegrated granite, and now the pro- 

 cess of soil formation begins. The crop of the second 

 year will be more vigorous, having the advantage 

 of pulverized mineral matter, from which it will 

 more easily and abundantly obtain its mineral con- 

 stituents, and the advantage also of rotting veget- 

 ation, which yields carbonic acid, to say nothing of 

 the other products of its decomposition. Carbonic 

 acid being freely absorbable by water is carried by 

 the sap, as it ascends to the leaf where it ia de- 

 composed under the influence of sunlight, just the 

 same as if it had been observed directly by the under 

 surfaces of the leaf. The crop therefore gets a part 

 of its carbon in the form of carbonic acid directly 

 from the atmosphere by the agency of its leaves, 

 and the remainder in the same form from the soil 

 by the agency of the roots. But it is obvious that 

 the carbon obtained by the second crop from the 

 soil was obtained by the first crop from the atmo- 

 sphere, so that the primary and original source of 

 all the carbon of the jilants is the atmosphere. By 

 exact parity of reasoning we will be able, even when 

 a century has elapsed, and a true soil has been 

 formed on the underlying granite, to trace back all 

 the carbon of each succeeding crop to the carbonic 

 acid of the atmosphere as its primary source, though 

 ■A'lth each successive year, the amount obtained from 

 the soil has been increasing, from the admixture 

 with it of the dead vegetation of preceding years. 

 Our co'ichision then is that plants, as they grow 

 under ordinary circumstances, obtain their carbon 

 partly from the atmosphere and partly from the soil, 

 but that the carbonic acid of the atmosphere is thfi 

 primary and original source of supply. 



The exhaustive e?cperiments of Bouseingault, con= 

 ducted through a series of seven years, show that 

 plants, as they ordinarily grow obtain § of their 

 carbon directly from the atmosphere, and J from 

 the soil, This experiment, perhaps unsuited for our 

 pnges is very simple, is easily understood, and irre- 

 sistibly carries the foregoing conclusion. 



If it be objected that the amount of carbonic acid 

 in the atmosphere seems too small to furnish to all 

 plants growing on the globe 3 of their carbon, the 

 objection will vanish as soon as we calculate the 

 absolute w'eight of the carbonic acid of the entire 

 atmosphere. The amount of carbonic acid in the 

 atmospliere is exceedingly small; the amoimt of 

 carbonic acid in the atmosphere is enormously large, 

 A word explains the seeming contradiction; relat= 

 ively small, absolutely large. 



One hundred gallons of air contain 4-100 of a gal- 

 lon of carbonic acid. One gallon of air then contains 

 1-2500 of a gallon of carbonic acid. But as carbonic 

 acid in oun and a half timet as Ueavy tis air, ^9 



