66 HEAT. 



CHAPTER VII. 



VAPORIZATION. 

 149. FORMATION OF VAPORS. While 



Do vapors . . 



form at all melting, or the conversion of a solid into 



temperatures? a liquid) occurs on ly w hen the solid is 



heated up to a certain fixed point, the conversion of a 

 liquid into a vapor takes place at all temperatures. 

 Thus water is always passing off into vapor from the 

 surface of the ocean, and from the moist earth. 



150. VAPORS TRANSPARENT. All vapors 



What is the 



appearance of are perfectly transparent, like the atmo- 

 sphere. If water be boiled in a flask, it 

 will be found that the steam within the flask is as 

 transparent as air. The steam thrown from a locomo- 

 tive would be invisible if it remained steam We 

 should hear its roar, but see nothing. 



151. DENSITY OF VAPORS. Vapors are 



/ the density 



of vapors uni- of all degrees of density. The vapor of 

 water may be as thin as air, or, again, al- 

 most as dense as water itself. 



152. ELASTICITY OF VAPORS. All va- 



lllustrate the 



elasticity of pors are elastic, like air. Steam, like air, 

 vapors. ft compressed in a cylinder, with a close 



fitting piston, by a heavy weight, would expand again, 

 and force the piston out, as soon as the weight were 

 removed. The force with which a vapor expands, or 

 strives to expand supposing the weight not removed, 

 is called its elastic force or tension. 



