TEMPERATURE. 



313 



the ice would sink and destroy everything living in the 

 water. The entire body of water would soon become a 

 solid mass which the heat of summer could not wholly 

 melt, for, as we shall soon see, water has little power to 

 carry heat downward. As it is, in even the coldest winters, 

 the mass of water in our northern lakes remains at a tem- 

 perature of 4C., the colder water floats upon the warmer 

 layer, ice forms over all, and protects the living things 

 below. 



49O. Expansion of Gases. The expansion of 

 gases may be shown by partly filling a bladder with cold 

 air, tying up the opening, and placing the bladder near 

 the fire. The expanded air will fill the bladder. Through 

 the cork of a bottle pass a small glass tube about a foot 

 long. Warm the bottle a little between the hands and 

 place a drop of ink at the end of the tube. As the air 

 contracts the ink will move down the tube and form a 

 frictionless liquid index. 

 By heating or cooling the 

 bottle the index may be 

 made to move up or down. 

 If a closed flask having a 

 delivery tube terminating 

 under water be heated, 

 some of the expanded air 

 will be forced to escape, 

 and may be seen bubbling 

 through the water. By 

 "collecting over water" 

 the air thus driven out, 

 it may be accurately 

 measured. (Fig. 243.) 



14: 



FIG. 243. 



