POND LIFE. 105 



float. Steel is seven times heavier than water ; never- 

 theless, in this experiment we see a piece of steel 

 floating: on the surface of water. 



O 



Experiment 2. Take a needle that you have caused 

 to float on the water and wet it thoroughly. Now 

 you will be unable to make it float so long as it is 

 wet. This experiment indicates that one of the con- 

 ditions of floating; of a heavy object is that it shall 

 not be wet by the liquid. 



Experiment j.- -Take needles of different sizes or 

 short pieces of different sizes of wire, and, having 

 them perfectly dry, try to make them float. In this 

 experiment it is well to lay the piece of wire across 

 the tines of a fork and lower it gently into the water. 

 It will be found that only the smaller needles or 

 pieces of wire will float. This experiment indicates 

 that a second condition of the floating of a heavy 

 object is that it shall be comparatively small. 



Experiment 4- -Take a glass of water and place it 

 on a stand so that you can look through the side of 

 the glass at the surface of the water both from above 

 and below. Place a needle upon the water, the 

 larger the needle the better, provided it will float. 

 By studying the floating object carefully it will be 

 seen that it rests in a little hollow with sloping sides, 

 the surface of the water being indented as if it were 

 covered with a membrane stretched across it. 



There are many other interesting phenomena that 

 are explained by a knowledge of the action of the 

 surface-film of liquids, but their study belongs to the 

 subject of physics ; only so much has been introduced 

 here as is necessary to understand what we will see 

 in our study of pond life. 



