SCIENCE OF COMMON THINGS. 139 



Salt and snow produce intense cold reason of. 



886 Are salt and snow really colder than snow ? 



Yes, many degrees ; and by dipping your hand into 

 the mixture first, and into snow afterwards, the snow 

 will seem to be comparatively warm. 



887 Why do salt and snow mixed together produce intense cold ? 



The salt and ' snow are both solids ; when they are 

 mixed, the salt causes the snow to melt by reason of its 

 attraction for water, and the water formed dissolves 

 the salt : so that both pass from the solid to the liquid 

 condition, and a large quantity of heat is absorbed. 

 As this heat is derived from that which previously ex- 

 isted in the solids themselves in a sensible state, its 

 abstraction occasions a reduction of temperature. 



888 How is heal produced by mechanical action f 



1. By percussion / 2. By friction / and 3. By con- 

 densation. 



88Q What is meant by percussion f 



The shock produced by the collision of two bodies ; 

 as when a blacksmith strikes a piece of iron on his 

 anvil with his hammer. 



00 O Why does striking iron make it red hot f 



Because it condenses the particles of the metal, and 

 makes the latent heat sensible. 



801 What is meant by friction f 



The act of rubbing two things together, as the Indians 

 rub two pieces of wood together to produce fire. 



80S How do savages produce fire by merely rubbing two pieces of dry 

 wood together ? 



They take a piece of dry wood, sharpened to a point, 

 which they rub quickly up and down & flat piece till a 

 groove is made ; and the dust (collected, in this groove) 

 catches fire. 



80S Why does the dust of the wood catch fire by rubbing f 



Because latent heat is developed from the wood by 

 friction. 



804 Do not carnage wheels sometimes catch fire f 



Yes ; when the wheels are dry, or fit too tightly, or 

 revolve very rapidly. 



