THE AMMONIA OF THE SOIL. 245 
crystalline solid. Double decomposition has taken place 
between the two salts under the influence of water. If, 
again, the carbonate of lime on the filter be reunited to 
the liquid filtrate and the whole be evaporated, it will be 
found that when the water has so far passed off that a 
moist, pasty mass remains, the odor of ammonia becoines 
evident again—carbonate of ammonia, in fact, escaping by 
volatilization, while sulphate of lime is reproduced. It is 
a general law in chemistry that when a number of acids 
and bases are together, those which under the circum- 
stances can produce by their union a volatile body will 
unite, and those which under the circumstances can form a 
solid body will unite. When carbonic and sulphuric acids, 
lime and ammonia, are in mixture, it is the circumstances 
which determine in what mode these bodies combine. In 
presence of much water carbonate of lime is formed be- 
cause of its insolubility, water not being able to destroy 
its solidity, and sulphate of ammonia necessarily results 
by the union of the other two substances. When the wa- 
ter is removed by evaporation, all the possible compounds 
between carbonic and sulphuric acids, lime and am- 
monia, become solid; the compound of ammonia and car- 
bonie acid being then volatile, this fact determines its 
formation, and, as it escapes, the lime and sulphuric acid 
can but remain in combination. 
To apply these principles: When carbonate of ammo- 
nia is brought into the soil by rain, or otherwise, it tends 
in presence of much water to enter into insoluble combi- 
nations so far as is possible. When the soil becomes dry, 
these compounds begin to undergo decomposition, provid- 
ed carbonates of lime, magnesia, potash, and so:la, are 
present to transpose with them; these bases taking the 
place of the ammonia, while the carbonic acid they were 
united with, forms with the latter a volatile compound. 
In this way, then, all soils, for it is probable that no soil 
exists which is destitute of carbonates, may give off at the 
