GENERAL PROPERTIES OF ALUMINA. 
7 
than silica, and very rarely if ever enters into the constitution of vegetables, so as to form 
an essential part of their tissues. If its functions as a part of the soil are also taken into 
account, it is found to differ essentially from the former. Silica, as already observed, 
preserves the soil in an open or free state : alumine, on the contrary, makes it consistent, 
compact and impervious. The functions of this body are confined to the soil. It is ne¬ 
cessary to observe that it is not the pure alumina of chemists which forms so large a 
portion of what is termed clay soils, but a silicate of alumina. Alumina, however, exists 
in two conditions even here : one of which is easily soluble, and may be called free alu¬ 
mina ; the other is comparatively insoluble and fixed, and before it can be fully dissolved 
even in the strong acids, requires fusion with the alkalies or alkaline earths. 
The facts most important to be known in the history of alumina, are the following : First, 
its insolubility in water; secondly, its neutral action on plants ; thirdly, the impervious 
condition it imparts to soil ; fourthly, its uniform state of fineness ; and, fifthly, its high 
affinity for water. It never forms a gravel, nor is ever found in coarse particles like the 
gravel of silica. This great degree of fineness is seen when it is diffused in water : weeks 
are sometimes required for the water in which it is diffused to become clear. 
It is unnecessary to dwell longer upon the properties of alumina : the most important 
facts, though briefly alluded to, are sufficient to answer the objects in view. 
The two foregoing bodies, silica and alumina, form the basis of all soils, and to their 
presence the soils owe their main and permanent characters. The presence of other 
elements produce but trifling modifications. Organic matter exerts a greater influence 
than any of the earths. The body or basis being thus constituted, the remaining elements, 
along with soluble silica, may be regarded as food, or as matter out of which the frame 
work of the various tissues is formed. Although this is without doubt true, yet it is proper, 
before leaving the subject, to say of soil composed entirely of those bodies termed nutritive, 
that if composed of either singly, it would form but a barren waste. A pure limestone 
soil, or one mixed with silica and subcarbonate of lime, would be infertile. It is not a 
matter of indifference what constitutes the basis or substratum of soils. This must be a 
peculiar medium having certain relations to water and other bodies, which, if not secured 
by and in their physical or mechanical properties, would render nugatory the labors of the 
farmer. 
Most of the earths possess low absorbent properties. Alumina, however, when pure, or 
as a silicate, ranks high in this important particular. The rapid absorption of ammonia 
by clay, renders-it a receptacle or reservoir of this element. Odors, too, may be expelled 
from clothes and other articles, by burying the articles in aluminous soil. 
LIME. 
Caustic lime, or the subcaustic hydrate of lime, is never an element of soil. Its affinity 
for carbonic acid permits it to exist only as a carbonate, or some neutral salt of lime. 
