556 



Miscellaneous. 



starch. The only new thing which he has advanced in it, is the 

 view that carbonic acid is assimilated by plants by means of their 

 leaves ; and this proposition must, for the present at least, be regard- 

 ed as quite unfounded. In proof that carbonic acid dissolved in 

 water is introduced into plants by their roots, I shall here produce 

 two calculations, which rest on very different foundations, and yet 

 agree in so remarkable a way with one another, and with other facts, 

 that I think many will be inclined to put some value on them. 



I. According to Hales, a sunflower, 3^ ft. high, evaporated every 

 day 1 lb. and 4 oz.; therefore in 120 days, during its vegetation, 

 150 lbs. = 3 cubic feet. These, saturated, contain 3 cubic feet of 

 carbonic acid. I shall allot to every plant 2 square feet of soil. 

 There would therefore be on the acre, assumed by Dr. Liebig, 

 20,000 of such plants. 3 cubic feet of carbonic acid have the 

 weight of about 5 oz. Therefore all the plants absorb with the 

 water 6,250 lbs. of carbonic acid, or 1,600 lbs. of carbon. The 

 production of carbon over the surface of an acre is, according 

 to Liebig's calculation equal to 1,029 lbs. There remain, therefore, 

 still 630 lbs., which have been employed in the growth of the roots, 

 &c., as also in that of the leaves which have withered during the 

 period of vegetation. 



II. The supposed acre, if we assume one foot for the depth of the 

 earth which contributes to the growth of the plants, contains 40,000 

 cubic feet. ; or, if we assume the specific gravity of the earth, at 

 an average at 2.0 it contains 400,000 lbs. of earth. These again 

 contain 40,000 lbs. of humus, or 1 per cent. They absorb from 

 the atmosphere, in 24 hours, 40,000 lbs. of water, and in the assum- 

 ed period of vegetation, i. e. in 120 days, 4,800,000 lbs. of water. 

 To this is to be added the average quantity of rain ; viz. 600,000 lbs. 

 and then we obtain, 5,400,000 lbs. of water. The sunflowers, which 

 Hales used for his experiments, have only on the lower side of their 

 leaves stomates by which evaporation takes place. Let us suppose 

 that the surface of the plant is 38 square ft. but that only 2 square ft. 

 are occupied by the stomates, by which evaporation is effected ; then 

 we find that that portion of the surface of the plant by which 

 the evaporation goes on is equal to the surface of the earth from 

 which it draws its nourishment. If we suppose that the evaporation 



