66 Chemistry. [Lecture 26. 



part is more transparent than the under, and the 

 lower parts receive almost all their heat in a se- 

 condary manner by reflection from the earth. 

 But it still remains to show why the lowest part 

 continues warmest, and does not rise as in the 

 other fluids. This fact was partly explained in 

 a former lecture. Air is a fluid very different 

 from water : it is very compressible. The atmo- 

 sphere may be supposed to be composed of very 

 eccentric layers, the lowest layer being the 

 densest. The lower strata, then, are rarefied a 

 little by heat, but are compressed by the supe- 

 rior ; and the heat is preserved by this compres- 

 sion of the superior air on the surface of the 

 earth, where it is intended to act. 



The coldness of the air seems to condense 

 the vapours, and causes them to fall in showers, 

 upon which the life of vegetables depends. This 

 shows the use of planting the superior parts of a 

 country. As green-houses made of glass receive 

 the heat transmitted through it, but confine the 

 air when heated ; so, on the contrary, the ground 

 that is quite exposed and naked has its heat so 

 quickly carried away by scorching winds that it 

 is rendered barren. 



3dly. FLUIDITY, as was formerly mentioned, 

 is one of the most general effects of heat. Not 

 only solid bodies, we find, may be rendered fluid 

 by heat alone, but even those bodies which ge- 

 nerally appear fluid owe their fluidity to the 

 caloric they contain, which being sufficiently 



