175 
eartlis. Those which contain the greatest quantity of sand retain 
it the shortest, and those which contain the greatest quantity of 
alumina, retain it longest. The first is a dry, the second a wet 
soil. Lime and magnesia are intermediate between these two 
extremes: they render a sandy soil more retentive of moisture, 
and diminish the wetness of a clayey soil. It is evident there- 
fore, that l)y mixing together proper |)roportions of these four 
earths, we may form a soil of any degree of dryness and mois- 
ture that we please. 
But w hatever be the nature of the soil, its moisture must de- 
pend in general upon the quantity of rain which falls. If no 
rain at all fall, a soil, however retentive of moisture it be, must 
remain dry ; and if rain were very frequently falling, the soil 
must be open indeed, if it be not constantly wet. The propor- 
tion of the different earths in a soil, therefore, must depend 
upon the (piantity of rain which falls. In a rainy country, the 
soil ought to be open ; in a dry country it ought to be retentive 
of moisture. In the first, there ought to be a greater propor- 
tion of sand, in the second of clay.” 
178 Air indispensable to Vegetation. “That air is neces- 
sary for the vegetation of (ilants has been completely jiroved; 
and that a considerable portion, at least of the carbonaceous 
matter which they contain, is absorbed from the air, has been 
rendered probable by the experiments of chemical philosophers. 
Three articles which furnish nourishment to plants are supplied 
by the atmosphere; namely, carbonic acid, oxygen, and mois- 
ture; but it has been disputed how far plants are capable of 
absorbing carbonic acid, without the assistance of the soil on 
which they usually vegetate. 
Mr. Hassenfratz analyzed the bulbous roots of hyacinths, in 
order to discover the quantity of water, carbon, and hydrogen, 
which they contained ; and by repeating the analysis on a num- 
ber of bulbs, he discovered how much of these ingredients was 
contained in a given weight of the bulb. He analyzed also 
kidney-beans and cress seeds in the same manner. Then he 
made a number of each of these vegetate in pure water, taking 
the precaution to weigh them before hand, in order to ascertain 
the precise quantify of carbon which they contained. The 
plants being then placed, some within doors, and others in the 
open air, grew and flowered, but produced no seed. He after- 
wards dried them, collecting with care all their leaves and every 
1S3 auctaeicm. 
