THEORY OF VEGETATION. 37 



In these qualities are found the mechanical relations 

 between earths and vegetables. To the divisibility 

 of the former it is owing that the latter are enabled 

 to push their roots into the earth ; to their density, 

 that plants maintain themselves in an erect posture, 

 rise into the air, and resist the action of winds and 

 rains ; and to their power of absorbing and holding 

 water, they owe the advantage of a prolonged ap 

 plication of moisture, necessary or useful to vege 

 table life. But, besides performing these important 

 offices, there is reason to believe that the earths COP 

 tribute to the food of vegetables. This opinion rests 

 on the following considerations and experiments : 



1. If earths do not contribute directly to the food 

 of plants, then would all soils be alike productive ; 

 or, in other words, if air and water exclusively sup- 

 ply this food, then would a soil of pure sand be as 

 productive as one of the richest alluvion. 



2. Though plants may be made to grow in pound- 

 ed glass or in metallic oxydes, yet is their growth 

 in these neither healthy nor vigorous : and, 



3. All plants, on analysis, yield an earthy pro- 

 duct ;* and this product is found to partake of the 

 earth that predominates in the soil producing the 

 analyzed plant. This important fact is proved by 

 De Saussure. 



FIRST EXPERIMENT. 



Two plants (the pinus abies) were selected, the 

 one from a calcareous, the other from a granitic 

 soil, the ashes of which gave the following pro- 

 ducts : 



Granitic soil. Calcareous soil. 



Potash .... 3.60 ... 15 

 Alk. and mu. sulphates 4.24 . . 15 



magnesian earth is not only barren itself, but the cause of barren- 

 ness in other soils in which it may abound, unless saturated with 

 carbonic acid. See Bosc, Tenant, and Davy. 



* Davy says this never exceeds one fiftieth of the whole pro- 

 duct. 



* A 1 i^-r"^/* 



oJLU/ ^b 



