l8 HEAT 



imparts less and less warmth to the water, which slowly cools 

 to a lower temperature. 



Man is a poor judge of temperature. Man is not an accu- 

 rate judge of temperature. Ice water seems comparatively 

 warm after eating ice cream, and yet we know that ice water 

 is by no means warm. A room may seem warm to a person 

 who has been walking in the cold air, while it may feel decidedly 

 cold to one who has come from a warmer room. If the hand 

 is cold, lukewarm water feels hot, but if the hand has been in 

 very hot water and is then transferred to lukewarm water, the 

 latter seems cold. We see that the sensation or feeling 

 of warmth is not an accurate guide to the temperature 

 of a substance ; and yet until 1592, one hundred years 

 after the discovery of America, people relied solely 

 upon their sensations for the measurement of tempera- 

 ture! The necessity, as well as the convenience, of 

 having some accurate means of determining the tem- 

 perature of substances led to the invention of the 

 thermometer, an instrument whose operation depends 

 upon the fact that most substances expand when 

 heated and contract when cooled. 



The thermometer. The modern thermometer con- 

 sists of a glass tube at the lower end of which is a 

 bulb filled with mercury or colored alcohol (Fig. 7). 

 After the bulb has been filled with the mercury, it is 

 heated a little above the boiling point of water and 

 the tube is then sealed at the top. As the mercury 

 J Makinga co ls, ** fells an d leaves an unoccupied space or 

 thermom- vacuum above it. The thermometer is then placed 

 in a beaker of water and the water is heated by a 

 Bunsen burner. As the water becomes warmer and warmer 

 the level of the mercury in the tube steadily rises until the 

 water boils, when the level remains stationary (Fig. 8). A 



