A FREEZING MACHINE. 



365 



bottle to be frozen is put into the cavity. The cold is so great, in conse- 

 quence of the transformation of the liquid ammonia into a gas, that it freezes 

 the water in any vessel put into the receiver. The ammonia can be re- 

 converted into liquid and back again, so no loss is occasioned by the process, 

 which is rapid and simple. This is how great blocks of ice are produced in 

 water-bottles. 



The one important point upon which care is necessary is the raising of 

 the temperature. If it be elevated beyond 130 C, the pressure will be too 

 great, and an explosion will occur. 



The abundant formation of ammonia from decaying animal matter 

 is evident to everyone, and depends upon the presence of moisture to a 

 great extent. Chloride of ammonia is called sal-ammoniac, and the 

 carbonate of ammonia crystallize om the alkaline liquid produced by the 

 distillation of certain animal matter. The compounds of ammonia are 

 easily recognized by a certain sharp taste. They are highly valuable 

 remedial agents, acting par- 

 ticularly upon the cutaneous 

 system, and when taken in- 

 ternally, produce the effect of 

 powerful sudorifics. Their 

 volatility, and the facility with 

 which they are expelled from 

 other substances, render them 

 of great importance in che- 

 mistry, and peculiarly fit them 

 for the purposes of many 

 chemical analyses. The am- 

 monia compounds display a 

 remarkable analogy to the 

 corresponding combinations of 

 potash and soda. The com- 

 pounds of ammonia are highly important in their relation to the vegetable 

 kingdom. It may be assumed that all the nitrogen of plants is derived 

 from the ammonia which they absorb from the soil, and from the sur- 

 rounding atmosphere. 



The similarity of ammonia to the metallic oxides has led to the 

 conjecture that all its combinations contain a compound metallic body, which 

 has received the name ammonium (NH 4 ) ; but no one has yet succeeded in 

 its preparation, although by peculiar processes it may be obtained in the 

 form of an amalgam. 



Ammonias, in which one or more atoms of hydrogen are replaced by 

 basic radicals, are termed Amides, or Amines. 



Fig. 360. Carre's rcfi 



