370 
PHYSIOLOGY. 
BOOK II. 
similar manner, except that only two ounces of nitrate of 
strontian were dissolved in ten gallons of distilled water, 
although the whole of that quantity was expended upon them, 
a minute examination demonstrated that the stems contained 
no trace whatever of strontian, although a small portion ap- 
peared to be present in, or at least adherent to^ the roots. By 
other experiments it was ascertained, that the strontian was 
not in these cases first received into the system, and after- 
wards rejected through the roots ; for when the roots of a 
Pelargonium were divided into two nearly equal bundles, one 
of which had its extremity immersed in a glass containing a 
weak solution of nitrate of strontian, the other in one con- 
taining pure distilled water, after the lapse of a week the water 
in the second glass was tested, but no strontian could be dis- 
covered 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 quality by some 
primary law of nature, although their amount may depend 
upon the more or less abundant supply of the principles pre- 
sented 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 indispens- 
able to the health of particular plants; as, for example, of 
Corn. Corn cannot remain in health, unless it has the power 
of attracting fluid silex from the earth, and of consolidating 
it in its epidermis. It is to be supposed, that the presence of 
alkaline principles in the soil is necessary to render the sili- 
ceous matter soluble ; therefore, to exhaust a soil of alkaline 
principles would be to render it unfit for the support of Corn ; 
and, consequently, alkaline principles may be considered nu- 
tritive in regard to Corn : and so of other things. 
Again,Thaer and Boussingault both agree in considering the 
efficiency of manures dependent in a great measure upon their 
animalised nature, or their power of adding nitrogen to vege- 
