ORIGIN AND FORMATION OF SOILS. 111 
Isomorphism.—In 1830, Mitscherlich, a Prussian phi- 
losopher, discovered that a number of the elementary 
bodies are capable of replacing each other in combination, 
from the fact of their natural crystalline form being identic- 
al; they being, as he termed it, isomorphous, or of like 
shape. Thus, magnesia, lime, protoxide of iron, protoxide 
of manganese, which are all protoride-bases, form one 
group, each of whose members may take the place of the 
other. Alumina (Al, O,) and oxide of iron (Fe, O,) be- 
long to another group of sesquioxide-bases, one of which 
may replace the other; while in certain combinations 
silica and alumina replace each other as acids. 
These replacements, which may take place indefinitely 
within certain limits, thus may greatly affect the composi- 
tion without altering the constitution of a mineral. Of 
the mineral amphibole, for example, there are known a 
great number of varieties; some pure white in color, con- 
taining, in addition to silica, magnesia and lime; others 
pale green, a small portion of magnesia being replaced by 
protoxide of iron; others black, containing alumina in 
place of a portion of silica, and with oxides of iron and 
Manganese in large proportion. All these varieties of 
amphibole, however, admit of one expression of their 
constitution, for the amount of oxygen in the bases, no 
matter what they are, or what their proportions, bears a 
constant relation to the oxygen of the silica (and alumina) 
they contain, the ratio being 1: 2. 
If the protoxides be grouped together under the gen- 
eral symbol MO (metallic protoxide,) the composition of 
the amphiboles may be expressed by the formula MO SiO,,. 
In pyroxene the same replacements of protoxide-bases 
on the one hand, and of silica and alumina on the other, 
occur in extreme range. (See analyses, p. 112.) The gen- 
eral formula which includes all the varieties of pyroxene 
is the same as that of amphibole. The distinction of am- 
phibole from pyroxene is one of crystallization. 
