TEMPERATURE. 



309 



B f of 18, or 10 centigrade degrees above the freezing point. Hence 

 the reading will be 10 C. 



(6.) Suppose that we desire to find the equivalent Fahrenheit 

 reading for 45 C. This temperature is 45 centigrade degrees above 

 the freezing point, or 81 Fahrenheit degrees above the freezing 

 point. Hence the reading will be (81 + 32 =) 113 F. (See Fig. 239.) 



(c.) The centigrade thermometer is the most convenient and is 

 adopted in all countries as the standard scale for scientific reference. 

 Like the metric system, its general use in this country is probably 

 only a question of time. 



Note. It is desirable that this class be provided with several 

 " chemical " thermometers ; i. e., thermometers having the scale 

 marked on the glass tube instead of a metal frame. 



82. Differential Thermometer. Leslie's dif- 

 ferential thermometer (Fig. 240) shows the difference in 

 temperature of two neighboring places by 

 the expansion of air in one of two bulbs. 

 These bulbs are connected by a bent glass 

 tube containing some liquid not easily 

 volatile. It is an instrument of simple 

 construction (See Appendix, M.) and great 

 delicacy of action, but has been largely 

 superseded by the thermopile and galvan- 

 ometer (414,391). 



483. Expansion. Heat consists 

 generally of molecular vibrations. What- PIG. 240. 

 ever raises the temperature of a body 

 increases the energy with which the molecules of that 

 body swing to and fro. These molecules are too small ( 5), 

 and their range of motion too minute to be visible, and we 

 must call upon our imaginations to make good the defect 

 of our senses. We must conceive these invisible molecules 

 as held together by the force of cohesion, yet vibrating 

 to and fro. The more intense the heat, the greater the 



