IN PHYSICS. 227 



snow which first falls upon the mountain-top into the 

 solid ice of the glacier, are very well illustrated, as Helm- 

 holtz has remarked, in the manufacture of the school- 

 boy's snow-ball or snow-man. Very cold snow is always 

 light and flaky, and cannot be made by the pressure of 

 the hands into a cohesive mass ; in order to succeed in 

 that operation, snow is always employed which is already 

 at the melting-point, or only so far below this temperature 

 that the warmth of the hand suffices to bring it to the 

 required temperature, and then, by dint of pressure and 

 moulding, an icy ball may be easily produced. So with 

 the formation of the glacier ice. A process of almost 

 simultaneous melting and freezing goes on among the 

 under layers of snow, and under an immense and ever- 

 constant pressure from the weight of the snow above j 

 thus solid ice is formed. That this ice conforms itself to 

 the various windings, constrictions, and dilatations of its 

 rocky channel during its downward march is a fact not 

 less familiar than wonderful." 



74. Why is the sheet of zinc under a stove so apt to 

 become puckered ? 



When zinc cools after expansion it does not return 

 quite to its former dimensions, and so becomes " puck- 

 ered," as it is called. 



75. Why does a mist gather in the receiver of the air- 

 pump as the air becomes rarefied ? 



"The remaining air, cooled by rarefaction, absorbs 

 heat from the invisible vapor in combination with it, and 

 renders the water visible. The mist may be removed by 

 continued action of the machine, or by readmitting the 

 normal quantity of air." 



(See Arnott's Physics, p. 448.) 



