16 THE PRINCIPLES OF AGRICILTURE. 



Solids, Liquids, and Gases. — Matter exists in one of three 

 states : either as a solid, a Uqiiid, or a gas. The same 

 substance may assume one of these forms at one time, 

 and another at another time. 



The state is supposed to depend ni)on the degree of 

 attraction by which its molecules are bound together. 



When they are firmly united, the substance is a solid. 

 When the force that binds them is weaker, allowing them 

 to move freely upon one another, it becomes a liquid. 

 When the force is entirely overcome, it becomes a vapor 

 or gas. The molecules then fly apart, tending to occupy 

 as much space as possible. 



The state of a substance is partly dependent upon the 

 temperature. 



Heat tends to overcome the attraction, and so to change 

 a substance to the liquid or gaseous form. A moderate 

 amount of heat will change a block of ice, first to water 

 and then to vapor. A higher temperature will produce 

 a similar effect upon other solids. 



Heat, l)y overcoming the force that draws molecules 

 together, increases the distance between them, and so 

 increases the space which they occupy and the size of 

 the body. So, on the other hand, a low temperature al- 

 lows the molecules to come nearer together, and renders 

 the body smaller. 



There are a few exceptions to the general rule that 

 heat expands and cold contracts. When water is cooled 

 enough to freeze, or become a solid, it is crystallized ; 

 that is to say, its molecules arrange themselves in cer- 

 tain forms which require more space than if they were 

 packed closely together. The same is ti-ue of a few other 

 substances when changing from the liipiid to the solid 

 state. 



