LESSONS IX PHYSICS. 27 



mercury which has been confined in a glass tube. A scale of 

 degrees is attached to the tube, of which the following are the 

 most important points : 212 degrees, which indicates the tem- 

 perature of boiling water; 32 degrees, which indicates the tem- 

 perature of freezing water; and zero, indicating a temperature 

 32 deegrees below that of freezing water. At 39 degrees below 

 zero mercury freezes, and to indicate lower temperatures alcohol 

 thermometers are commonly used. 



The above are the figures on the Fahrenheit scale. On the 

 Centigrade scale, zero marks the freezing point, and 100 the boil- 

 ing point. Other scales are sometimes used. 



To measure higher temperatures, an instrument called a pyrom- 

 eter is used, in which some metal or one of the permanent gases 

 serves to indicate the intensity of the heat. To measure delicate 

 changes in temperature an air thermometer is sometimes used. 

 In our experiments for showing the expansion of water and air 

 by heat, the tubes, with the water in one case and the air in the 

 other, formed a thermometer, needing only a scale for completion. 



The principal sources of heat for the earth are the sun and 

 stars. Chemical action, as combustion, fermentation, and the 

 vital processes, is a source of heat; so is mechanical action, as 

 friction, percussion, etc. Illustrations of heat from these various 

 sources will be familiar to everyone. 



Heat is assumed to be the motion of the molecules of bodies. 

 An imponderable but highly elastic substance, called ether, is 

 supposed to fill the spaces between the molecules of bodies and all 

 spaces throughout the universe not otherwise occupied. The 

 vibrating molecules of the heated body cause vibrations or waves 

 in this ether. These waves spreading out in all directions cause a 

 more rapid movement of the particles of bodies which they meet, 

 thus causing more intense heat. In fact, heat does not pass from 

 point to point as heat, but rather as energy which becomes heat 

 in the bodies acted upon by the waves of ether. This transfer- 

 ence of heat through waves of ether is called radiation. The 

 intensity of heat by radiation varies; first, as the intensity of 

 heat in the source; second, inversely, as the square ofthedistance 



