86 PHYSIOGRAPHY. [chap. 



is a compound of nitrogen with a gas called hydrogen, 

 which will be described in the next chapter; it is necessary 

 however to refer briefly, in this place, to the composition 

 of ammonia, since this gas, though present in only such 

 minute proportion, furnishes to plants a large part of 

 their nitrogen, just as the carbonic acid gas supplies them 

 with their carbon. Traces of 7iitric acid, the substance 

 known commonly as aquafortis, are occasionally found in the 

 atmosphere, especially after thunderstorms ; this nitric acid 

 readily combines with the ammonia to form nitrate of am- 

 monia, the presence of which may frequently be detected in 

 rain water. Sulphuretted hydrogen, an offensive gas given 

 off during the putrefaction of animal and vegetable matter, 

 may also be often found in the air ; and a few other gases 

 are sometimes present, especially in air taken from the 

 neighbourhood of large towns. Nor should mention be 

 omitted of the organic germs, which constantly float in 

 the atmosphere, but of which it is beyond our present pur- 

 pose to speak. As to the watery vapour, which is ever 

 present in the air, it is unnecessary to say anything here, 

 since the subject was fully discussed in the last Chapter. 



This watery vapour differs from the other constituents of 

 the atmosphere principally in the ease v/ith which it may be 

 condensed or liquefied. Hence it is generally called a 

 vapour rather than a gas. Yet there is really but little 

 distinction between the two classes of bodies ; a vapour 

 being nothing more than an easily-condensible gas. Steam, 

 for example, is liquefied by a comparatively slight reduction 

 of temperature ; carbonic acid and a number of other gases 

 require a great reduction of temperature or a great pressure, 

 or even a combination of cold and pressure, in order to 

 assume the liquid form ; whilst six gases have hitherto re- 

 sisted all attempts at liquefaction, and are therefore called 



