88 SCIENCE OF COMMON THINGS. 



Mist on windows. Insensible perspiration. 



falls at sunset, and chills the window-glass with which 

 it comes in contact. 



559 How does this account for the mist and water on a window f 



As the warm vapor of the room touches the cold 

 glass it is chilled and condensed into mist, and the mist 

 (collecting into drops) rolls down the window-frame in 

 little streams of water. 



560 Does the glass of a ivindow cool down more rapidly than the air 

 of the room itself f 



Yes ; because the air is kept warm by fires and by 

 the animal heat of the people in the room ; in conse- 

 quence of which the air of a room suffers very little 

 diminution of heat from the setting of the sun. 



561 Whence arises the vapor of a room f 



The air of the room always contains vapor ; vapor 

 also arises from the breath and insensible perspiration of 

 the inmates, from cooking and the evaporation of water. 



563 What is meant by " the insensible perspiration f " 



From every part of the human body an insensible 

 and invisible perspiration issues all night and day, not 

 only in the hot weather of summer, but also in the 

 coldest days of winter. 



563 If the perspiration be both insensible and invisible, hoiu is it known 

 that there is any such perspiration ? 



If you put your naked arm into a clean, dry glass 

 tube, the perspiration will condense on the glass like 

 mist. 



564 Why is a tumbler of cold water made quite dull with mist, when 

 brought into a warm room ? 



Because the hot vapor of the room is condensed upon 

 the cold tumbler, with which it comes in contact, and 

 changes its invisible and gaseous form into that of dew. 



565 Why does breathing on a glass make it quite dull f 



Because the cold glass condenses the invisible vapor 

 contained in warm breath, and converts it into dew. 



566 Why are the walls of a house covered with damp in a sudden 

 thaw? 



Because the walls (being thick) cannot change their 



