CURRENTS IN AIR AND WATER 93 



engines are composed in like manner of fine particles of condensed 

 vapor of water. 



There are many points of difficulty in this with the children, 

 from former notions picked up here and there. It is hard to un- 

 derstand that steam is invisible, as are many of the vapors in the 

 preceding lessons (of gasoline, ether, etc.). But these very diffi- 

 culties give opportunities for questions, which they discuss with 

 each other, for the settling of which they may devise experiments. 

 Do not be in too great a hurry to have it all taught. We may re- 

 member that what we wish of our material is, that it may give 

 just those questions which it is possible for the children to work 

 on. We should give them a chance to work at these questions 

 when we find them. For those pupils who are ready for it, the 

 thermometer may be used, and the gradual rise of temperature 

 observed until the water boils, when it will be seen that it stands 

 At 212 degrees Fahrenheit, whether in the boiling water, or in the 

 steam just above it (not in the fog formed outside). Alcohol may 

 be used to show that other liquids have a lower boiling point 



Condensation. 



The phenomenon of condensation will constantly come up in 

 the foregoing experiments and is, of course, to be noted. The 

 iodine vapor condenses in crystals on the cool sides of the test 

 tube, or if poured on a cool piece of glass. A lump of camphor 

 heated in a test tube in the same way will evaporate and condense 

 on the sides of the tube. Many experiments may be devised to 

 further illustrate the results of evaporation and condensation of 

 water. It must be seen that such high degrees of heat as for boil- 

 ing is not needed for evaporation. 



A glass vessel, partly filled with water, and the mouth closed, 

 will show water constantly on its inner surface, coming from 

 condensing the vapor arising from the water. 



The condensation of water on the cold surface of a plate of 



