214 Mr. W. Murton Holmes on 
As a rock-forming agent silica takes the part of an acid, hence 
it is frequently called silicic acid; and in combination with 
various bases, such as alumina, potash, soda, magnesia, &c., 
forms a large series of important minerals. 
It is, however, about silica in the uncombined state that I 
propose to offer a few remarks now. 
In the free state it occurs most abundantly in the crystalline 
condition as represented by quartz, and is one of the principal 
constituents of granite and gneiss. Less abundantly it occurs 
in a semi-crystalline state as flint and chalcedony; and it also 
occurs in a colloid or amorphous state, containing from five to 
twelve per cent. of water, forming the varieties of opal. 
In the crystalline condition silica is insoluble in water at 
ordinary temperatures and pressures; but in the hydrous con- 
dition it is to a certain extent soluble in water, and the presence 
of alkaline carbonates materially increases the solubility. When, 
however, it is exposed to the combined action of pressure and 
heat, the quantity dissolved is much larger. 
Such conditions obtain in the geysers of Iceland and of the 
Yellowstone Park in the United States, and the quantity of 
silica held in solution by the water as it issues from these has 
been found to be 5-097 in 10,000 parts.* On being discharged 
from a geyser, the water forms a series of pools, in the neigh- 
bourhood of which a certain amount of vegetation, particularly 
of alge, is able to exist, and it is by the action of these organisms, 
rather than by simple cooling and evaporation, that the dissolved 
silica is precipitated upon surrounding objects, forming what is 
known as a sinter. In this way shells and other objects become 
imbedded, as in the specimen exhibited. One of the sinter-beds 
in the Iceland geyser region is said to be two leagues long, a 
quarter of a league wide, and a hundred feet thick. Enormous 
beds of similar material have been deposited in the Yellowstone 
Park geyser region. 
If to a solution of silicate of soda a little hydrochloric acid be 
added, the acid will combine with the soda, setting free the 
silica in a gelatinous condition, hence the term colloid applied 
to the non-crystalline form. 
In much the same way living protoplasm has frequently 
the property of being able to separate silica from solution, 
and in many cases of setting it free and absorbing it from 
earthy silicates, such as clay, either held in suspension in 
* These geysers are springs which at certain intervals throw up large 
volumes of boiling water. The water underground, under the pressure of a 
long column of water, attains a temperature higher than that of ordinary 
boiling water; but when the elastic pressure becomes strong enough to 
overcome the pressure of the superincumbent column, the water is ejected 
with considerable violence. 
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