354 HOW CROPS FEED. 
proportions, extremely important to the soil, and espe- 
cially so when existing in combinations admitting of the 
remarkable changes which have come under our notice. 
That we cannot decide as to the precise composition of 
the zeolitic compounds which may exist in the soil, is plain 
from what has been stated. We have the certainty of 
their analogy with the well-defined silicates of the miner- 
alogist, which have been termed zeolites, an analogy of 
chemical composition and of chemical properties ; we know 
further that they are likely to be numerous and to be in 
perpetual alteration, as they are subjected to the influence 
of one and another of the salts and substances that are 
brought into contact with them; but more than this, at 
present, we cannot be certain of. 
Physical agencies in the phenomena of absorption.— 
While the absorption by the soil of potash or other base 
is accompanied by a chemical decomposition, which Way, 
Rautenberg, Heiden, and Knop’s researches conclusively 
connect with certain hydrous silicates whose presence in 
the soil cannot be doubted, it has been the opinion of 
Liebig, Brustlein, Henneberg, Stohmann and Peters, 
that the real cause of the absorption is physical, and is 
due to simple surface attraction (adhesion) of the porous 
soil to the absorbed substance. Brustlein and Peters have 
shown that bone and wood-charcoal, washed with acids, 
absorb ammonia and potash from their salts to some ex- 
tent, and after impregnation with carbonate of lime to as 
great an extent as ordinary soil. While the reasons al- 
ready given appear to show satisfactorily that the ab- 
sorbent power of the soil, for bases in combination, re- 
sides in the chemical action of zeolitic silicates, the facts 
just mentioned indicate that the physical properties of the - 
soil may also exert an influence. Indeed, the fixation of 
Sree bases by the soil may be in all cases partially due to 
this cause, as Brustlein has made evident in case of am- 
monia (Boussingault’s Agronomie, ete., T., II, p. 153). 
