184 SCIENCE OF COMMON THINGS. 



Why it is chilly during a thaw. Cause of hoar-frost. 



the air, etc., to melt the ice ; in consequence of which 

 the heat of the air is greatly reduced. 



1212 To what extent can the temperature be reduced by the liquefaction 

 of a mixture of the two solids, snow and salt ? 



By means of a mixture of equal weights of common 

 salt and fresh snow (or pounded ice), a steady temper- 

 ature of nearly 40 below the freezing point of water 

 can be maintained for hours. 



1213 How much heat is required to melt ice f 



The conversion of a cube of ice, three feet on the 

 side, into water at 32, would absorb all the heat pro- 

 duced by the combustion of a bushel of coal. 



1214 Why does the frost of winter make the earth in spring loose and 

 friable f 



Because the water absorbed by the earth in warm 

 weather, expanding by the frost, thrusts the particles 

 of earth apart from each other, and leaves a chink or 

 crack between. 



1215 Show the wisdom of the Creator in this arrangement. 



These cracks in the earth let in air, dew, rain, and 

 many gases favorable to vegetation. 



1216 Why are delicate trees covered with straw in winter f 



Because straw (being a non-conductor) prevents the 

 sap of the tree from being frozen. 



1217 What is hoarfrost f 



There are two sorts of hoarfrost : 1. Frozen dew ; and 

 2. Frozen fog. 



1218 What is the cause of the ground hoarfrost, or frozen dew ? 



Yery rapid radiation of heat from the earth ; in 

 consequence of which the surface is so cooled down, 

 that \\, freezes the dew condensed upon it. 



1219 Why is hoarfrost seen only after a very clear night ? 



Because the earth will not have thrown off heat 

 enough by radiation io freeze the vapor condensed upon 

 its surface, unless the night be very clear indeed. 



1220 What is the cause of that hoarfrost which arises from frozen fog f 



