HEAT. 



47 



point to be at 32, and the boiling-point at 180 degrees above it, or at 

 212. (Fig. 16.) Degrees of temperature below the zero point of 

 either instrument are designated by , i. e. y minus. 



100 







FIG. 16. 



....BE 



212 



As 100 degrees centigrade are equivalent to 180 degrees Fahrenheit, it follows 

 that 1 degree C. = 1.8 degree F., or 1 degree F. = 

 \% degree C. In converting the degrees from one 

 scale to the other it must be remembered that the 

 zero point of Fahrenheit is 32 degrees below the 

 zero on the centigrade scale. Consequently, in 

 converting the reading on a Fahrenheit scale into 

 centigrade degrees, 82 degrees must be deducted, 

 or, vice versa, be added, before the calculation can 

 be made. In other words, to convert Fahrenheit 

 into centigrade : Subtract 32 and divide by 1.8 ; to 

 convert centigrade into Fahrenheit : Multiply by 

 1.8 and add 32. 



Recording or self-registering thermome- 

 ters. One kind of these instruments is so con- 

 structed as to show the highest, the other the 

 lowest temperature to which the thermometer had 

 been exposed from the time it was last adjusted. 

 The physician's fever thermometer is a maximum 

 recording thermometer with a scale ranging usu- 

 ally from 94 to 112 F. These thermometers 

 have near the bulb a constriction in the tube 

 which breaks the column of mercury when it con- 

 tracts. Consequently the thin column of mercury 

 retains its position on cooling, but may be forced 

 back through the constriction into the bulb by 

 shaking the instrument. 



p-p. 



32 



Centigrade. Fahrenheit. 

 Thermometric scales. 



Absolute temperature. If differences 



of temperature are due to faster or slower motion of molecules, we 

 can well imagine that there is a limit in both directions. Where the 

 limit is to be found for rapidity of motion is unknown, but good rea- 

 sons lead us to believe that we know the point at which molecular 

 motion ceases. 



One of the reasons which lead us to believe in the correctness of 

 this statement is the Law of Charles, above mentioned. In accord- 

 ance with this law, it follows that if a mass of air at C. is heated, 

 its volume is increased -3-7-3- of the original volume for every degree 

 its temperature is raised. At 273 C. its volume is consequently 



