CURRENTS IN AIR AND WATER 9* 



Evaporation. 



A lesson on evaporation could begin as follows: On a sheet 

 of clean glass place a drop each of: water, gasoline, alcohol and 

 glycerine or molasses, and have the children watch the result 

 The gasoline soon disappears, later the alcohol, the water much 

 more slowly , while the glycerine persists indefinitely. Questioning 

 and study of this will bring out clearly the fact of evaporation be- 

 fore the class and something of its nature and how different liquids 

 behave in this respect. Place a few drops of gasoline at the bottom 

 of a glass tumbler closed with a covering. The gasoline disap- 

 pears from the bottom. It can not escape and thus it is seen that 

 the vapor must be transparent like the air with which it is min- 

 gled. A lighted match placed at the mouth of the tumbler proves 

 the presence of the vapor of gasoline by the slight explosion and 

 burning. 



Bisulphide of carbon, used so much in the destruction of squir- 

 rels, evaporates readily, gives a vapor so heavy that it can be 

 poured from one glass to another. It also explodes when lighted 

 in a glass. 



The rate of evaporation of water in the conditions of the 

 school room may be observed by the use of a jar of water, the 

 height of the water being noted at definite intervals. 



With a few crystals of iodine placed in the bottom of a test 

 tube and gently heated over the flame of a lamp a beautiful purple 

 vapor can be produced. Crystals of iodine may be obtained at 

 any drugstore and this experiment is excellent as giving a vapor 

 that can be seen, one that is very heavy and can be poured out. 

 It thus by its different appearance makes clearer the conception of 

 a vapor. It is also an example of a solid changing to a vapor. 



That the process of evaporation is a cooling one, that it 

 uses up heat, can be shown by placing a bit of cloth moistened 

 with water on the bulb of a thermometer and seeing the fall of 

 the mercury as the water evaporates. With the use of a rapidly 



