THE ASH OF PLANTS. 119 
acid is formed. This compound is a gas that dissolves with great avidity 
in water, forming a liquid which has a sharp, sour taste, and possesses 
all the characters of an acid. 
The muriatic acid of the apothecary is water holding in solution several 
hundred times its bulk of chlorhydric acid gas, and is prepared from com- 
mon salt, whence its ancient name spirits of salt. 
Chlorhydric acid is the usual source of chlorine gas. The latter is 
evolved from a heated mixture of this acid with peroxide of manganese. 
In this reaction the hydrogen of the cbhlorhydric acid unites with the 
oxygen of the peroxide of manganese, producing water, while chloride 
of manganese and free chlorine are separated. - 
4HCl + MnO, = MnCl, + 2H,O + 2Cl. 
When chlorine dissolved in water, is exposed to the sun-light, there 
ensues a change the reverse of that just noticed. Water is decomposed, 
its oxygen is set free, and chlorhydric acid is formed, 
H,O + 2Cl = 2HCl + O. 
This reaction probably takes place when the germination of seeds is 
hastened by chlorine. The oxygen thus liberated is doubtless the real 
agent which excites growth in the sleeping germ. 
The two reactions just noticed are instructive examples of the differ- 
ent play of affinities between several elements under unlike cireum- 
stances. 
Chiorhydrie acid, being volatile, does not occur in the ashes of plants, 
nor probably in the plant itself, unless, as may possibly happen, it is 
formed in, and exhales from the vegetation, as it sometimes does from 
the mud of salt marshes, (p. 118.) Chlorhydric gas is found in voleanic 
emanations. 
This acid is a ready means of converting various metals or metallic 
oxides into chlorides, and its solution in water is a valuable solvent and 
reagent for the purposes of the chemist. 
Todine, Sym. I, at. wt. 127.—This interesting body is a black solid at 
ordinary temperatures, having an odor resembling that of chlorine. Gent- 
ly heated, it is converted into a violet vapor. It occurs in sea-weeds, 
and is obtained from their ashes. It gives with starch a blue or purple 
compound, and is hence employed as a test for that substance, (p. 64.) 
It is analogous to chlorine in its chemical relations. Itis not known to 
occur in sensible quantity in agricultural plants, although it may well 
exist in the grasses of salt-bogs, and in the produce of soils which are 
manured with sea-weed. 
Bromine and Fluorime myy also exist in very small quantity in 
plants, but these elements require no further notice in this treatise. 
SILICON AND ITS COMPOUNDS. 
Silicon, Sym. Si, aé. wt. 28.—This element, in the free 
state, is only known to the chemist. It may be prepared 
