14 HEAT 



5. Temperature. When an object feels hot to the touch, 

 we say that it has a high temperature ; when it feels cold to 

 the touch, that it has a low temperature; but we are not 

 accurate judges of heat. 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 some 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 

 will seem cold. We see that the sensation or feeling 

 of warmth is not an accurate guide to the tempera- 

 ture 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 

 temperature. Very hot substances cannot be touched 

 without injury, and hence inconvenience as well as 

 the necessity for accuracy 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. 



6. The Thermometer. The modern thermometer 

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

 a bulb filled with mercury or colored alcohol (Fig. 

 8). After the bulb has been filled with the mer- 



FTP 8 



Making a cury, it is placed in a beaker of water and the water 

 thermom- } s nea t.ed by a Bunsen burner. As the water be- 



eter. 



comes warmer and warmer the level of the mercury 

 in the tube steadily rises until the water boils, when the level 

 remains stationary (Fig. 9). A scratch is made on the tube to 

 indicate the point to which the mercury rises when the bulb 

 is placed in boiling water, and this point is marked 212. 



