SCIENCE OF COMMON THINGS. 91 



What is rain ? Why falls in drops. 



and 2. The vapor is frozen upon the ground before it 

 ean rise from the earth, and becomes hoar-frost. 



581 What is rain? 



Rain is the vapor of the clouds or air condensed and 

 precipitated to the earth. 



58J3 In what manner is the vapor of the air condensed so as to form 

 rain ? 



i When two or more volumes of humid air differing 

 considerably in temperature unite, the several portions 

 in union are incapable of absorbing the same amount 

 of moisture that each could retain if they had not 

 united. The excess of moisture, if very great, is pre- 

 cipitated as rain ; if in slight amount, it appears as 

 clouds, fogs, or mists. 



583 Upon what law does this condensation of vapor and formation of 

 rain depend f 



Upon the law that the capacity of the air for moist- 

 ure decreases in a greater ratio than the temperature. 



58*4 Why does rainfall in drops ? 



Because the vapory particles in their descent attract 

 each other ; and those which are sufficiently near unite 

 and form into drops. 



The size of the rain-drop is increased according to the rapidity with 

 which the vapors are condensed. 



585 Why does not the cold of night always cause ram f 



Because the air is not always near saturation ; and 

 unless this be the case, it will be able to hold its vapor 

 in solution, even after it is condensed by the chilly 

 night. 



580 Why does a passing cloud often drop rain ? 



Because the cloud (travelling about on the wind) 

 comes into contact with something that chills it; 

 and its vapor being condensed, falls to the earth as 



58 T Can the air absorb moisture at all temperatures, and retain it in 

 an invisible state f 



It can ; and this power of the air is termed its 

 capacity of absorption. 



