FISHES. LIGHT. 363 



Q. How are FISHES able to ASCEND to the 

 SURFACE of water ? 



A. Fishes have an air-bladder near 

 their abdomen: when this bladder is 

 filled with air, the fish increases in size; 

 and (being lighter) ascends through the 

 water to its surface. 



Q. How are fishes able to DIVE in a minute 

 to the BOTTOM of a stream ? 



A. They expel the air from their air- 

 bladder ; in consequence of which, their 

 size is diminished, and they sink instantly. 



CHAPTER XXVII. 

 LIGHT. 



Q. What is LIGHT ? 



A. Rapid undulations of a fluid called 

 ether, striking on the optic nerve of the 

 eye. (Seep. 46.) 



The heat of fire or of the sun sets the atoms of matter 

 in motion ; and these atoms, striking against the fluid 

 ether, cause it to undulate. 



ia3 



