HEAT. 55 



temperature of our body is kept normal in heated weather. A thin 

 glass beaker, containing ether and resting on some water on a glass 

 plate, may quickly be frozen to the plate by passing a rapid current 

 of air through the ether. 



The number of calories of heat required to vaporize 1 gramme of 

 any liquid at a given temperature is called its latent heat of vaporiza- 

 tion. The latent heat of steam at 100 C. is 535.9 calories, that is, 

 it requires as much heat to convert 1 gramme of water at 100 C. 

 into steam at 100 C., as would raise 535.9 grammes of water 1 C. 

 in temperature. Steam has the largest latent heat of all known sub- 

 stances, hence, its value in warming houses, etc., by the steam-heat- 

 ing process. 



Influence of pressure on state of aggregation. We have seen 

 that the volume of a substance, and, more especially, of a gas, depends 

 upon pressure and temperature, an increase of pressure or decrease of 

 temperature causing the volume to become smaller. We learned also 

 that liquids may be converted into gases, and that this conversion 

 takes place at a certain fixed temperature, called the boiling-point. 

 This point, however, changes with the pressure. An increased pres- 

 sure will raise, a decreased pressure will lower, the boiling-point. 



Thus, water boils at the normal pressure of one atmosphere at 100 C. (212 

 F.), but it will boil at a lower temperature on mountains in consequence of the 

 diminished atmospheric pressure. If the pressure be increased, as, for instance, 

 in steam-boilers, the boiling-point will be raised. Thus, the boiling-point of 

 water under a pressure of two atmospheres is at 122 C. (251 F.), of five atmo- 

 spheres at 153 C. (307 F.), of ten atmospheres at 180 C. (356 F.). A differ- 

 ence of pressure of 10 millimeters from the normal atmospheric pressure (760 

 mm.) produces a difference of 0.36 C. in the boiling-point of water, 100 C. 



QUESTIONS. What is the view in regard to the nature of heat? What is 

 meant by sensible heat and latent heat? What is the law in regard to expan- 

 sion of gases when heated? Explain the construction of a thermometer and 

 the principle on which it depends. What Fahrenheit temperature corresponds 

 to 50 C., to 130 C., to 40 C.? What Centigrade temperature corresponds 

 to 167 F., to 311 F., to 14 F. ? What is meant by absolute temperature? 

 Give a definition of the following : Calorie, specific heat, conduction, convec- 

 tion, and radiation of heat. Define melting and state how the melting-point is 

 determined ; define latent heat of fusion. State the difference between evapo- 

 ration and boiling. What is distillation and sublimation? What is meant by 

 latent heat of vaporization ? What is the influence of pressure on the boiling- 

 point of liquids ? 



