30 PRACTICAL LESSONS IN SCIENCE. 



for example, boils at a higher temperature than pure water, 

 while that which contains air boils at a much lower temperature 

 than that which contains none. In the salt water the attract- 

 ive force is stronger than in the fresh water, and more heat 

 is necessary to overcome this force, while in the water contain- 

 ing air the attractive force is less, and less heat is necessary to 

 raise it to the boiling point. The temperature of the boiling 

 point also depends upon the nature of the vessel in which the 

 water is boiled. If the adhesion between the water and the vessel 

 is strong it requires more heat to overcome it, and vice versa. 

 Thus, water will boil at a lower temperature in an iron vessel 

 than in a glass vessel, because the force of adhesion is stronger 

 between water and glass than between water and iron. The tem- 

 perature of the boiling point of water also depends upon the 

 pressure. This pressure may be due to the atmosphere, or to 

 some force which may be brought to bear upon it by artificial 

 means. Whatever may be the cause of pressure the effect is to 

 raise the boiling point. Whatever serves to decrease the press- 

 ure lowers the temperature of the boiling point. It is well 

 known that water boils at a lower temperature on the top of a 

 mountain than at its base. It does so because the pressure of 

 the air upon it is less. At some high mountain stations the tem- 

 perature of boiling water is so low that it is of little value for 

 cooking purposes. 



When heat falls upon a body a portion is thrown back, or 

 reflected, from the surface. Dense bodies, as the metals, with 

 smooth surfaces, are the best reflectors of heat as well as light. 

 About midday in summer, when the sun is shining, it is warmer 

 on the south side of a building than in an open field, showing 

 that even rough surfaces will reflect heat. Heat is reflected ac- 

 cording to the same laws observed in the reflection of light, but 

 they can be illustrated more easily in connection with the study 

 of light. 



When heat falls on the surface of a body from any source, one 

 portion is reflected, while another portion penetrates the body, 

 passing from molecule to molecule, as in the case of conduction. 



