THE SUBSTANCES OF THE EARTH. 



15 



is sufficient to overcome the force of cohesion by which 

 the molecules are attracted to one another, and they are 

 separated or the rubber is broken. 



The Porosity of Matter. All matter is more or less 

 porous. This is not only true of loose substances like 

 soil, but also of more solid substances, like wood and 

 iron. The pores in the latter are, like atoms and mole- 

 cules, too small to be seen. 



n c w 



Cross-section of Wood, magnified, showing Pores. 

 , the bark ; C, the cambium layer; W, wood. 



It is believed that neither atoms nor molecules ever 

 remain in absolute contact with each other, but that there 

 are always spaces between them. Through these spaces 

 atoms and molecules of other substances are able to pass. 



A certain amount of salt and sugar may be dissolved 

 in water without increasing the volume of water. The 

 molecules of salt and 'sugar occupy the vacant spaces 

 between the molecules of water. 



Under heavy pressure water has been forced through 

 the pores of iron. A piece of iron may be made smaller 

 by hammering. Its molecules are then driven nearer 

 together. 



A bottle filled with gas will hold as much of another 

 kind of gas as if it were empty. 



