14 EXPERIMENTAL GENERAL SCIENCE 



probably never will, scientists have devised means of weighing 

 and otherwise measuring them, and thus have proved the 

 truth of what has long been known as the molecular theory of 

 matter. 



The molecular theory enables us to understand how it is 

 possible for matter to change its form. Water, for instance, 

 may exist as a solid (ice), a liquid (water), or a gas (steam), and 

 in each of these states, the same quantity has a different size. 

 If matter were continuous and not composed of many small 

 particles, it is difficult to see how it could expand and contract 

 in this way. Various experiments may be performed to illus- 

 trate the molecular structure of matter; thus, if salt is slowly 

 sifted into a test-tube of water, fine bubbles of air are seen to 

 rise to the surface. These air bubbles are composed of mole- 

 cules of air which were crowded out from between the mole- 

 cules of water by the salt. Again, water at high pressure may 

 be forced through sheets of solid steel or other metals, and car- 

 bon dioxide gas easily passes through red-hot iron. A single 

 drop of soapy water contains so many molecules that when 

 blown into a soap bubble more than a foot in diameter, there 

 are enough to form a continuous film throughout. 



20. States of Matter . Water is not the only substance that 

 may exist in three states solid, liquid, and gaseous. Many 

 other substances change in the same way when conditions are 

 favorable. Each state has certain characteristics peculiar to 

 it. Solids always have a shape of their own and resist with 

 considerable force any effort to change it. When solids are 

 changed to liquids, however, they invariably take the shape of 

 any vessel in which they happen to be, with the upper, free, 

 surface level. Gases have neither shape nor size (volume) of 

 their own. They fill any space open to them, and push out- 

 ward with equal force in all directions. For this reason, they 

 cannot be kept in an open vessel like solids and liquids. If 

 left uncovered, the molecules at once begin to fly away. If 



