SCIENCE OF COMMON THINGS. 199 



Air cool after a rain. Production and nature of steam. 



131V Why does bread after the lapse of a few days become dry and 

 stale f 



Because the moisture contained in it evaporates ; the 

 particles then shrink, and the whole mass becomes 

 hard. 



1318 Why is not the vapor of the sea salt f 



Because the salt is always left behind in the process 

 of evaporation. 



131O " AH the rivers run into the sea :" why is not the sea full f 



Because the quantity of water evaporated from the 

 surface of the sea is equal to the quantity poured into 

 it by the rivers ; therefore the sea is never full. 



13J3O Why is it frequently cooler after a rain f 



Because water which falls from the atmosphere soon 

 returns to it in the form of vapor, carrying with it, in 

 the latent form, a large amount of heat taken from 

 every object, thus moderating the temperature of the 

 earth, and refreshing the animal and vegetable creations. 



1331 Does evaporation take place from the surface of snow and ice f 



Yes, to a very considerable extent, even when the 

 temperature of the air is below the freezing point. 



13SS What is steam? 



The vapor of boiling water. 



1323 Is steam visible or invisible f 



Steam is invisible; but when it comes in contact 

 with the air (being condensed into small drops) it in- 

 stantly becomes visible. 



13S4 How do you know that steam is invisible f 



If you look at the spout of a boiling, kettle, you will 

 find that the steam (which issues from the spout) is 

 always invisible for about half an inch, after which it 

 becomes visible. 



1325 Why is the steam invisible for half an inch t 



Because the air is not able to condense it, as it first 

 issues from the spout ; but when it spreads and comes 

 in contact with a larger volume of air, the invisible 

 steam is readily condensed into visible drops* 



