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 chlorhydric acid unites with the 

 oxygen of the peroxide of manganese, producing water, while chloride 

 of manganese and free chlorine are separated. 



4 H Cl + Mn O 2 = Mn C1 2 + 2 H 2 O + 2 Cl. 



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 2 O + 2 Cl = 2 H Cl + 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 circum- 

 stances. 



Chlorhydric 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 volcanic 

 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. 



Iodine, 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. It is 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 Fluorine may 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, at. wt. 28. This element, in the free 

 j is only known to the chemist. It may be prepared 



