FORMS OF MATTER 3 



compressed into a much smaller space ; their chief characteristics can 

 be stated thus : Gases are easily compressible and expand indefinitely. 



Changes of state in same kind of matter. To the fact that 

 there are three kinds of material things, we must add another idea, 

 viz., that the same matter can exist in all three forms. The change 

 in the state of matter, whether from the solid to the liquid condition, 

 or from the liquid form to the gaseous state, is most easily brought 

 about by heat. Reverse changes, viz., from gas to liquid and from 

 liquid to solid, can be effected by cooling. 



The degree of heating required to bring about the above changes 

 varies very greatly with different substances. Iron must be heated 

 very much more than ice before it can become a liquid. Alcohol, 

 again, has to be cooled to a much greater extent than water before the 

 liquid condition gives place to that of a solid. 



When a substance has, as a result of heating, successively passed 

 through the solid, liquid, and gaseous states, then, if the conditions 

 are reversed and the gas is continuously cooled, the liquid form is first 

 reassumed, and subsequent cooling causes the liquid to change again 

 into the original solid. 



Sudden and gradual changes. The circumstances attending the 

 change from the solid to the liquid, or from the liquid to the gaseous 

 state, are not always the same as in the case of water. When a crystal 

 of iodine is heated, it appears to pass suddenly from the condition of 

 a solid to that of gas. Camphor is another instance of this sudden 

 transition from solid to vapour. When, on the other hand, sealing- 

 wax is heated, it very gradually passes into the liquid condition, and 

 may be obtained in a kind of transition stage neither true solid nor 

 true liquid. 



