218 
mass by saturating it with carbonic acid gas. 
The same decomposition must take place in very 
dilute solutions, although we cannot detect in 
them any separation of silica, which remains 
dissolved in the water. The decomposition of 
silicates by the combined action of water and of 
acids proceeds with a rapidity proportional to 
the quantity of alkalies contained in them. We 
find numerous examples in the inorganic king- 
dom of a continued and progressing process of 
decomposition of the silicates contained in the 
various kinds of rocks; this decomposition is 
effected by the action of carbonic acid and of 
water. 
“A consideration of the preceding observations 
shows clearly that porcelain clay or kaolin has 
been formed by the decomposing action of water 
on the silicates of potash and soda contained in 
felspar or felspathic rocks. Felspar may be 
viewed as a combination of silicate of alumina 
with silicate of potash; the last of which, being 
gradually removed by water, leaves behind the 
porcelain clay. It has been shown by Forcham- 
mer, that felspar may be decomposed by water 
of 302° F., and at a pressure corresponding to 
this temperature. The water becomes strongly 
alkaline, and is found to contain silica in solu- 
tion. The hot springs in Iceland possess a high 
temperature, and come from a great depth, where 
they must have been subjected to high pressure. 
Forchammer has shown by analysis that the 
water of these springs contains the constituents 
of soda felspars, and of magnesian silicates, 
minerals of very frequent occurrence in trap 
districts. There cannot be a doubt that a con- 
version of crystalline felspar into clay must be 
proceeding to a great extent at the bottom of 
these springs. Ordinary water containing car- 
bonic acid acts in precisely the same manner as 
water at a high temperature, and at a high pres- 
sure. Polstorf and Wiegmann boiled some white 
sand with a mixture of nitric and muriatic acids, 
and after completely removing the acid by wash- 
ing the sand with water, they exposed it thus 
purified to the action of water saturated with 
carbonic acid gas. After the expiration of 30 
days, this water was subjected to analysis, and 
was found to contain in solution, silica, carbonate 
of potash, and also lime and magnesia; thus 
proving that the silicates contained in the sand 
were unable to withstand the continued action 
of water containing carbonic acid, although the 
same silicates had resisted the short action of the 
aqua regia. 
“ Certain of the alkaline silicates found in na- 
ture contain in their crystalline state water in 
chemical combination. In this class are the 
zeolites, analcime, mesotype, sodalite, apophyllite, 
&c.; the felspars, properly so called, are always 
anhydrous. These silicates differ very much 
in their behaviour to acid reagents. When 
mesotype, or a mineral corresponding to it in 
composition, is kept in the state of a fine powder 
in contact with cold muriatic acid, it increases 
in bulk to a thick jelly. The mineral being ex- 
posed to the action of the acid at the ordinary | 
temperature, those constituents which are solu- 
ble in the acid are taken up by it whilst the 
greatest part of the silica remains undissolved. 
Labrador spar (calcareous felspar) behaves simi- 
larly when treated with acids; but the minerals 
adularia and albite (potash and soda felspars) 
are not attacked by acids under similar circum- 
stances.” 
Silica, under soluble conditions, is a compara- 
tively large ingredient in the food of agricultural 
plants, and plays an important part in the fer- 
tility of soils. The quantity of it in any one 
species of plant varies in different parts of the 
plant, in different varieties of the species, and 
upon different kinds of soil, The quantity of it, 
in the mean of six specimens of turnip bulbs, 
analysed by Professor Wey and Mr. Ogston, 
amounted to 0:34 lb. per ton; in the mean of 
six specimens of turnip-tops, to 1:73 lb. per ton; 
in the mean of six specimens of entire crops of 
turnips, to 0°55 lb. per ton; in the mean of three 
specimens of beet-bulbs, to 0°54 lb. per ton; in 
the mean of three specimens of beet-leaf, to 0°76 
lb. per ton; in the mean of three specimens of 
entire crops of beet, to 0:56 lb. per ton; in the 
mean of five specimens of carrot-roots, to 0:24 
lb. per ton; in the mean of three specimens of 
carrot leaves, to 4'46 lbs. per ton; in the mean 
of three specimens of entire crops of carrots, to 
1:22 Ib. per ton. The quantity of silica, accord- 
ing to the same authority, amounts to 0:61 lb. per 
ton in the tubers of Jerusalem artichoke, to 1:76 
lb. per ton in pease grown on chalk, to 0:84 lb. 
per ton in pease grown on clay, to 2:53 lbs. per 
ton in pease straw grown on chalk, to 5:04 lbs. 
per ton in the entire crop of pease grown on clay, 
to 1:94 lb. per ton in pease straw grown on clay, 
to 0°42 lb. per ton in beans grown on clay, to 
2°61 lbs. per ton in bean straw grown on clay, to 
3:17 lbs. per ton in the entire crop of beans grown 
on clay, to 60:0 lbs. per ton in wheat straw, to 
1721875 lbs. per ton in wheat chaff, and to 
162 oz. in every 28 bushels or 1,792 lbs. of wheat 
grain. Two important practical inferences from 
these facts, are, that the root crops bring up 
soluble silica from the deep parts of the soil, or 
assist the subjugation of crystalline silica under 
soluble condition in the higher parts of the soil, 
in preparation for the uses of the corn crops,— 
and that by far the major portion of all the solu- 
ble silica abstracted from the soil by a whole 
course of crops, may be returned to it in the 
leaves or tops of the root crops and especially in 
the straw and chaff of the grain crops. A main 
office which silica appears to perform is to give 
mechanical firmness and strength and resistive- 
ness to the parts and organs of plants which are 
most exposed to the risk of fracture or postration 
or other injury from the weather; and therefore 
it is scarcest in the pulp or farina of seeds and in 
