EXPANSION. 121 



oven, the air in the cup will begin to 

 expand, and drive every particle of juice 

 from under it; the pie dish, therefore, 

 will have a cup-full less room to hold its 

 fruit, than if the cup were taken out. 



Q. If the juice is driven OUT of the cup, why 

 is the CUP always FULL of JUICE, when the pie is 

 cut up ? 



A. Immediately the pie is drawn, the 

 air in the cup begins to condense again, 

 and occupy a smaller space ; in conse- 

 quence of which, there is no longer 

 enough air to fill the cup, and so juice 

 rushes in to fill up the deficiency. 



Q. Why does JUICE rush into the cup, because 

 the cup is NOT FULL of AIK ? 



A. As the external air presses upon 

 the surface of the juice, it rushes into the 

 cup unobstructed; as mercury rises through 

 the tube of a barometer through similar 

 pressure. 



