188 SCIENCE OF COMMON THING?. 



What is temperature ? Thermometers and pyrometers. 



changes with the heat to which it is exposed ; and since, 

 when subject to the same calorific influences, it always 

 has the same magnitude, these dilatations and contrac- 

 tions, which are the constant effects of heat, may be 

 taken as the measure of the physical cause that pro- 

 duced them. 



1343 What is the temperature of a body ? 



It is the actual state of a body at any moment, deter- 

 mined by a comparison of its magnitude with the heat 

 to which it is exposed. 



1344 What is a change of temperature f 



The change in magnitude which a body suffers by 

 changes in the heat to which it is exposed. 



1345 What are the instruments for measuring lieat called? 



Thermometers and pyrometers. 



1346 What is the difference between them f 



A thermometer is used for measuring moderate tem- 

 peratures ; w r hile the pyrometer is chiefly applied to de- 

 termine the more elevated degrees of heat. 



1347* What substances are best adapted for measuring the effects of 

 heat by their expansion and contraction f 



Liquids, above all other substances. 



1348 Why are liquids best adapted for this purpose f 



Because in solids the direct expansion by heat is so 

 small as to be seen or measured with difficulty ; in air 

 or gases it is too extensive and too liable to be affected 

 by atmospheric pressure ; but liquids are free from both 

 disadvantages. 



1349 What liquid is generally used for the construction of ordinary 

 thermometers f 



Mercury or quicksilver. 



035 O What metal is distinguished from aU others by its fluidity at 

 ordinary temperatures f 



Mercury or quicksilver. 



1351 Does mercury, like other metals, expand by heatf 



It readily expands or contracts with every variation 

 of temperature. 



