LIQUEFACTION. 61 



used. Heat expands the air and lengthens the column 

 downward, pushing the water before it, while cold has 

 the contrary effect. The temperature is thus indicated 

 by the height at which the water stands. 



135. ILLUSTRATION. The principle of 

 ^incTll *o/ the air thermometer may be illustrated as 

 the air ther- represented in the figure. A 



mometer. ^ , . , ,,, -. , , 



test-tube is half filled, and 

 then inverted in a glass of water without 

 allowing the water which it contains to 

 flow out. Heat applied to the tube will lengthen the 

 column of air by expansion. 



LIQUEFACTION. 

 136. SOLIDS BECOME LIQUIDS BY HEAT. 



flow do sohds 



become li- m On being heated up to a certain point, solids 

 quids? are jilted, or converted into liquids. 



Thus, at all temperatures below 32, water is solid 

 ice, but the moment it is warmed up to this point, 

 by change of weather or other means, it begins to 

 melt. The temperature at which this change occurs 

 is called the melting point. 32 is therefore the melt- 

 ing point of ice. The melting point of sulphur is 

 226, and of lead, 612. 



137. ALL SUBSTANCES ARE FUSIBLE. 



Are all sub- 

 stances fusi- All substances are fusible, or, in other 



words, may be melted ; but the melting 

 point of all is not definitely known. Thus carbon has 

 been fused by the heat of the galvanic battery, but it 

 is impossible to state the melting point in degrees. 



