68 SCIENCE OF COMMON THINGS. 



Varieties of gaseous bodies. Composition of the atmosphere. Air porous. 



a proportionally higher temperature ; if this pressure 

 is diminished, the amount of heat required is propor- 

 tionably less. 



414 How many kinds of aeriform or gaseous bodies exist in nature f 



Those which, under common circumstances of tem- 

 perature and pressure, are always in a gaseous state, 

 as common air ; and those which become gases chiefly 

 at high temperature, as steam, or vapor of water. 



415 Are all gases invisible or colorless like atmospheric air f 



Some gases possess color, but the greater number 

 are colorless and invisible. 



416 Of what is atmospheric air composed f 



Principally of two gases, oxygen and nitrogen, mixed 

 together in the following proportion : viz. one volume 

 of oxygen to four of nitrogen. 



It must not be forgotten that the air contains small quantities of other 

 gaseous substances also, as vapor of water, carbonic acid, and ammonia. 



41 7* Do the particles of which atmospheric air and other gaseous bodies 

 are composed, appear to have any cohesion between themselves f 



The ultimate particles of which air and other gases 

 are composed appear to be destitute of cohesion ; hence 

 air has a disposition not only to sink down and spread 

 out laterally, like liquids when unconfined, but also to 

 expand and rise upwards. 



418 Is the air porous f 



Yes ; in a very high degree. 



419 How do we know this fact ? 



Because air readily yields to pressure, and a great 

 bulk of it may be forced to occupy a very small space. 



4SO Is air also impenetrable f 



Yes ; beyond a certain limit it cannot be compressed. 



431 How much lighter is steam than ordinary air f 



Steam has but little more than half the weight of 

 atmospheric air ; and hence it rises and floats in the 

 air as a cork rises and floats in water. 



