48 CHEMICAL PHYSICS. 



doubled, and at 546 C. tripled. If a mass of air is cooled below 

 C., its volume is diminished ^^ of its volume for every degree 

 its temperature is lowered. Consequently, if its volume were to 

 continue to decrease at that rate until it reached 273 C., mathe- 

 matically speaking its volume would become nothing. In fact, 

 the air long before this low temperature had been reached would 

 cease to be a gas would first liquefy, then solidify, and at the tem- 

 perature of 273 C. would become a compact mass in which the 

 molecules were at absolute rest. This point of no heat is called the 

 absolute zero, and temperature reckoned from this point is called 

 absolute temperature. 



The fraction -^7-3- is called the coefficient of expansion for centigrade 

 degrees, while ^y is the coefficient of expansion for degrees of 

 Fahrenheit. The absolute temperature may be found by adding 273 

 to the reading on a centigrade thermometer, or 459 to the reading on 

 a Fahrenheit thermometer. 



While the temperature of absolute zero may never be obtainable by man, 

 so much successful work in the field of low temperatures has been done lately 

 that temperatures within 9 degrees of absolute zero have been observed. 



In chemistry and other fields, temperature is often indicated by such phrases 

 as red heat, white heat, etc. The following table gives approximately the 

 degrees corresponding to such expressions : 



Incipient red heat 525 C. (977 F.). 



Dark red heat 700 C. (1292 F.). 



Bright red heat 950 C. (1742 F.). 



Yellow heat 1100 C. (2012 F.). 



Incipient white heat . , 1300 C. (2372 P.). 



White heat 1500 C. (2732 F.). 



Mechanical equivalent of heat. While thermometers indicate 

 the intensity of heat, it is often desirable to measure heat quantities. 

 These determinations are based on the intimate relationship existing 

 between heat and mechanical or molecular motion, which are capable 

 of being converted one into the other. Thus, friction produces heat 

 and heat produces motion in the steam engine. Heat, through its 

 power to produce motion, can do work, and the amount of work it 

 can do depends on the quantity of heat. 



The unit of heat quantity now universally used in chemical and physiological 

 work is the calorie. It is the amount of heat consumed in raising 1 kilogram 

 of water from C. to 1 C. (or, approximately, 1 pound of water 4 degrees of 

 Fahrenheit). If this amount of heat could all be made to do mechanical work. 



