14 ELEMENTS OF AGRICULTURE 



CHAPTEE II.— Sunlight 

 (Continued) 



9. Solids, liquids, and Gases.— The changes which 

 water goes through when it evaporates to form cloudsj 

 and the freezing of clouds to hail or snow, show that 

 water may exist in three different forms : first as a 

 liquid, second as a gas, and third as a solid. Every- 

 thing we know is either a liquid, solid, or gas. Water 

 may be easily changed from one form to another, but 

 many substances are very difficult to change. Iron, a 

 solid, when heated very hot, melts to a liquid, and it 

 may even be heated so hot that it becomes a gas. Air, 

 under pressure, has been liquefied by intense cold, and 

 other wonderful changes have been made. As a rule, 

 heat causes substances to expand and melt, and if they 

 are heated sufficiently high they expand still more and 

 become gases. On the other hand, cold causes most sub- 

 stances to contract and change from gases to liquids, 

 and from liquids to solids. This expanding and con- 

 tracting of bodies is made use of in the thermometer, 

 which is a tube partly filled with mercury or alcohol. 

 All of the air is carefully taken out of the tube, the 

 mercury is then put in, and the tube sealed up. When 

 the mercury is heated it expands and rises in the tube; 

 when it cools off it contracts and falls. By this rising 

 and falling in the tube we measure the intensity of heat 

 or cold. 



