42 ABSORPTION OF RADIANT HEAT PAST i. 



senting the absorption of these four gases is only the 

 -g^th part of the unit in the preceding table. 



It appears from the preceding tables of comparative 

 absorption that chlorine, a highly-coloured gas with a 

 specific gravity of 2-45, has an absorptive power ex- 

 pressed by 39 under the pressure of one atmosphere, 

 while, at the same tension, hydrochloric acid, a chemical 

 compound of chlorine and hydrogen which is perfectly 

 transparent, with a specific gravity of only 1'26, has an 

 absorptive action amounting to 62, whence it appears 

 that the chemical change which renders chlorine more 

 transparent to light, makes it more opaque to obscure 

 heat. Again, bromine, which is far less permeable to 

 light than chlorine, and has a specific gravity of 5*54, 

 has an absorptive po.wer of 160 under a tension of one 

 inch ; while hydrobromic acid, which is perfectly trans- 

 parent to light, has an absorptive action for obscure 

 heat amounting to 1005. This is a striking instance of 

 transparency to light and opacity to heat being produced 

 by the very same chemical art. 



The enormous difference between the absorptive 

 power of compound and simple gases and vapours is 

 ascribed to their atomic structure ; in fact the radiant 

 and absorptive powers augment as the number of atoms 

 in the compound molecule augments. The three ele- 

 mentary gases are formed of simple atoms, the compound 

 gases and vapours consist of different kinds of atoms 

 chemically united into groups. Both are free to receive 

 the vibratory motions of the ether which constitute heat ; 

 but single atoms must produce a less effect than when 

 a number of them are united into a molecule. The 

 atoms are loaded by their chemical union, which offers a 

 greater surface of resistance to the vibrations of heat, 

 and renders the motion of the molecule more sluggish 

 and more fit to accept the slowly recurrent waves of the 

 obscure heat that strike upon it. 



