CHAP. IX. FOOD OF PLANTS. 309 



tion of nitrate of strontian, the other in one containing pure 

 distilled water, after the lapse of a week the water in the 

 second glass was tested, but no strontian could be discovered 

 in it, although a single grain in one pint would have been 

 readily detected. Hence it appears, " that plants do possess, 

 to a certain extent at least, a power of selection by their roots, 

 and that the earthy constituents which form the basis of their 

 solid parts are determined as to quantity by some primary 

 law of nature, although their amount may depend upon the 

 more or less abundant supply of the principles presented to 

 them from without." Linn. Trans, xvii. 266. 



It must be obvious that the exhaustion of soil by plants 

 means their having consumed all the nutritive particles that 

 it contains. Whether this means all particles that are ca- 

 pable of forming carbonic acid, is, however, not so certain : 

 it is highly probable that other matters are equally indispen- 

 sable to the health of particular plants ; as, for example, of corn. 

 Corn cannot remain in health unless it has the power of at- 

 tracting fluid silex from the earth, and of consolidating it in 

 its cuticle. It is to be supposed that the presence of alkaline 

 principles in the soil is necessary to render the siliceous mat- 

 ter soluble ; therefore, to exhaust a soil of alkaline principles 

 would be to render it unfit for the support of corn ; and, con- 

 sequently, alkaline principles may be considered nutritive 

 in regard to corn : and so of other things. 



Hence arises the very complicated nature of the theory of 

 manures, and the seeming impossibility of reducing it to any 

 fixed and intelligible laws. Ignorant as we are of most of the 

 more obscure phenomena that are attendant upon vegetable 

 life, unacquainted with the action of a large proportion of the 

 principles that the chemist discovers among the tissue of plants, 

 and incapacitated by our limited means of observation from 

 watching any except the most obvious and general properties 

 of living vegetable matter, we cannot expect, in such a state of 

 things, to arrive at any precise ideas as to what kind of food 

 or stimulants exercises the most energetic and wholesome in- 

 fluence upon plants. I accordingly feel no surprise at the 

 statement of a friend of mine, well known alike for his agri- 

 cultural skill, his chemical knowledge, and his remarkable 



X 3 



