22 TEE THREE CONDITIONS Of MATTER. 



the plate place a saucer so that when the saucer is filled 

 with water the plate may rest upon the liquid surface, the 

 scale-beam remaining horizontal. Carefully add small 

 weights to those in the scale-pan. Notice that the water 

 beneath the plate is raised above its level. Add more 

 weights until the plate is lifted from the water. Notice 

 that the under surface of the plate is wet. These mole- 

 cules on the plate have been torn from their companions 

 in the saucer. The weights added to the original coun- 

 terpoise were needed to overcome the tendency of the 

 water molecules to cling together. 



Note to the Pupil. After seeing a physical experiment, always ask 

 yourself, " What was the object of that experiment? What does it 

 teach?" Never allow yourself to look upon an experiment as being 

 simply entertaining ; thus reducing the experimenter, so far as you 

 are concerned, to the level of a showman. 



57. What is an Aeriform Body? An aeri- 

 form body is one whose molecules easily change 

 their relative positions, and tend to separate from 

 each other almost indefinitely. 



Atmospheric air is the best type of aeriform bodies. 



58. Gases and Vapors. Aeriform (having the 

 form of air) bodies are of two kinds, gases and vapors. 

 Gases remain aeriform under ordinary conditions, although 

 some, if not all, may be liquefied by intense cold and pres- 

 sure. Vapors are aeriform bodies produced by heat from 

 substances that are generally solid or liquid, as iodine or 

 water. They resume the solid or liquid form at the ordi- 

 nary temperature. 



59. Changes of Condition. The same substance 

 may exist in two or even three of these forms. Most 



