80 THE SUN'S GIFT OF HEAT 



bottle of sulphuric acid. There is an opening in the top of 

 the cylinder which is connected with the nozzle by means 

 of a short strong rubber tube. When the extinguisher is 

 to be operated, it must first be inverted. The acid falls out 

 of the bottle, and mingling with the solution of baking soda 

 rapidly generates carbon dioxide. The pressure of this 

 generating gas forces the solution mixed with the gas out 

 of the nozzle. Since carbon dioxide will not burn and is 

 considerably heavier than air, it helps the water to smother 

 the fire. Chemical fire-engines make use of this same gas. 



Measurement of Temperature. It has been seen (pages 

 64 and 65) that gases, liquids, and solids expand when 

 heated and contract when cooled. It has been 

 found that most substances expand uniformly 

 through ordinary ranges of temperature, so that 

 if this expansion or contraction is measured, we 

 are able to determine the change of temperature. 



Experiment 27. Slightly warm the bulb of an air 

 FIGURE 35 thermometer tube and place the open end in a beaker 

 half filled with inky water. (Figure 35.) Allow the 

 bulb to cool. The tube will become partly filled with the water. 

 When the bulb has become cooled to the temperature of its 

 surroundings, mark the end of the water column with a rubber 

 band. Grasp the bulb with the hand, thus warming the air in it. 

 The water column will run partially out of the tube back into the 

 beaker. Cool the bulb with a piece of ice or a damp cloth. The 

 water will come farther up in the tube than it did when simply 

 exposed to the air. We have here an apparatus for telling the 

 relative temperatures of bodies. 



Instruments arranged to show changes in temperature 

 by the amount of the expansion or contraction of certain 

 materials, are called thermometers. These may be gas, 



