12 



ELEMENTS OF GENERAL SCIENCE 



of water. Because there are two scales, it is always necessary, 

 when we write temperatures, to indicate which scale is used, 

 and thus we say that the boiling point is at 100 C. or 212 F. 

 Since both types of thermometers are in use, it is important 

 to be familiar with both of them and to be able to determine 

 the equivalent of a given reading of one 

 thermometer in terms of the other type of 

 thermometer. 



In the Fahrenheit scale the freezing tem- 

 perature is 32 above zero and the boiling 

 temperature is 212 above zero. Therefore 

 there are 180 between freezing and boiling, 

 according to the Fahrenheit system. In the 

 centigrade system the freezing temperature 

 is zero and the boiling temperature is 100. 

 Therefore 100 divisions on the centigrade 

 scale equal 180 divisions on the Fahrenheit 

 scale. One centigrade degree equals 1.8 

 Fahrenheit degrees. A temperature of 80 C. 

 is equivalent to 80 x 1.8 + 32, or 176 F. 



10. Some effects of expansion. It was 

 learned in a previous experiment that if the 

 air in a bottle is heated, the mouth of the 

 bottle remaining open, the air will expand 

 and some of it will escape. The air that 

 remains fills all the space that the whole 

 amount originally occupied, but plainly it 

 cannot weigh the same, for only a part of 

 the original air is in the bottle, and a part cannot weigh as 

 much as the whole. Careful measurement and weighing will 

 show that the weight of the bottle is less by exactly the weight 

 of the air which has escaped. We usually express this by say- 

 ing that heated air is lighter than cold air. We must remember, 

 however, that this is true only under certain conditions. For 

 instance, if we put a stopper in the bottle so that there can be 



FIG. 12. A mercurial 

 thermometer 



Both Fahrenheit and 



centigrade scales are 



shown 



