52 ITS EXISTENCE IN NATURE, AND SPECIAL PROPERTIES. 



almost universally, diffused, but is not known to form large deposits on 

 any part of the earth's surface, or to enter as a constituent into any 

 of the great mineral masses of which the crust of the globe is com- 

 posed. It exists most abundantly in a state of combination — in the 

 forms, for example, of muriate (sal ammoniac), of nitrate, and of carbon- 

 ate of ammonia. It frequently escapes into the atmosphere in an un- 

 combined state, especially where animal matters are undergoing decay, 

 but it rarely exists in this free state for any length of time. It speedily 

 unites with the carbonic acid of the air, with one or other of the numer- 

 ous acid vapours which are continually rising from the earth, or with 

 the nitric acid which is formed at the expense of the nitrogen and oxy- 

 gen of which the atmosphere consists. 



The influence of ammonia on vegetation appears to be of a very 

 powerful kind. It seems not only to promote the rapidity and luxu- 

 riance of vegetation, but to exercise a powerful control over the func- 

 tions of vegetable life. In reference to the nature and extent of this 

 action, into which we shall hereafter have occasion to inquire, there 

 are several special properties of ammonia which it will be of impor- 

 tance for us previously to understand. 



1°. It has a powerful affinity* for acid substances. Hence the 

 readiness with which it unites with acid vapours when it rises into the 

 atmosphere. Hence also when formed or liberated in the soil, in the 

 fold-yard, in the stable, or in compost heaps, it unites with such acid 

 substances as may be present in the soil, &c. and fornxs saline com- 

 pounds or salts. All these salts appear to be more or less liifluential in 

 the processes of vegetable life. 



2°. Yet this affinity is much less strong than that which is exhibited 

 for the same acids by potash, soda, lime, or magnesia. Hence if any 

 of these substances be mixed or brought into contact with a salt of am- 

 monia, the acid of the latter is taken up by the potash or lime, while 

 the ammonia is separated in a gaseous slate. Thus when sal ammo- 

 niac in powder is mixed with twice its weight of quick-lime, ammoni- 

 acal gas is liberated in large quantity. This is the method by which 

 pure ammonia is generally prepared ; and one of the many functions 

 performed by lime when employed for the improvement of land, espe- 

 cially on soils rich in animal and vegetable matter, is that of decompo- 

 sing the salts, especially the organic salts, of ammonia, — as will be 

 more fully explained when we come to treat at length of this important 

 part of agricultural practice. f 



3°. The salts which ammonia forms with the acids are all, like am- 

 monia itself, very solujsle in water. Hence two consequences follow. 

 First, that which rises into the air in the form of gas, and there com- 

 bines with the carbonic or other acids, is readily dissolved, washed out 



* By affinity is meant the tendency which bodies have to unite and to remain united or 

 combined. Thus ammonia forms a solid substance with the vapour of vinegar the moment 

 the two substances come into contact; they have, therefore, a strong tendency to unite, or 

 an affinity for each other. 



t See Lecture XVI. ^^On the use of lime." Owing to this property the action of lime upon, 

 compost heaps is often injurious, by causing the evolution of the ammonia produced during 

 the decomposition of the animal matters they contain. This escape of ammonia, even 

 when imperceptible by the sense of smell, is easily detected by holdinfi over the heap a fea- 

 ther dipped in vinegar or in spirit o!' salt (muriatic ?.cid), when vvliite fumes are immediate- 

 ly perceived if ammonia be prese j » 



