TEMPERATURE. 



the normal temperature of the human body was nearly constant, and 

 using a duodecimal scale he took this as 2 x 12 = 24. He found that 

 ice melted at 8 on this scale. These degrees being inconveniently large, 

 he quartered them, so that he had 0, 32, and 96 as fixed points. 

 Having verified Amontons' statement that water boils at a constant 

 temperature, he found that it was at 212 of his new quartered degrees. 

 The estimate of 96 for the human body was then found to be more 

 nearly 98 than 96. 



The Reaumur Scale. On this scale the fixed points are for melting 

 ice and 80 for boiling water. Reaumur used alcohol diluted with one- 

 fifth of water as the liquid in his thermometer, and each degree marked 

 an expansion of To ^ o0 of the volume of the liquid at 0. 



Conversion of Scales. If the same teinperatxire is indicated respectively 

 on the three scales by C, F, and R. then evidently the equations 



_F-32 

 100 ~ "180 



JR 



; 80 



enable us at once to convert from one scale to another. 



Marking" the Fixed Points. In this country the lower fixed point 

 is marked first. For this purpose the bulb of the 

 tube is immersed in a metal vessel containing a 

 mixture of small pieces of melting ice and air-free 

 distilled water. The vessel may also be surrounded 

 by melting ice to prevent the temperature of the 

 water in it rising above that of the ice. The ther- 

 mometer is placed so that the mercury rises just to 

 the top of the ice, and when it is steady a file mark 

 is made to show its position. 



The thermometer is then placed in a metal vessel, 

 the construction of which is indicated by Hg. 3, the 

 bulb and the tube being entirely surrounded by 

 steam. The thermometer is placed so that the 

 mercury rises just into sight and its final level, 

 which is only attained after some time, say a quarter 

 or half-an-hour, is again marked by a fine file. 



It is essential to read the barometer, as the 

 boiling temperature of water varies with the atmos- 

 pheric pressure, being only 99 at 733'2 mm. 

 (slightly below 29 inches). The point marked will 

 only be 100 on the rare occasions when the baro- 

 meter is at 760 mm., and, in general, allowance 

 must be made for the deviation. 



Pressure also affects the melting point of ice, 

 but the ordinary variations of pressure produce no 

 appreciable effect. Practically it is only important ~" 

 in ordinary thermometers to use pure water, and FIG. 3. Boiling-Point 

 to be sure that the ice is all at the melting Apparatus, 



temperature. Impurities lower the melting point, 



and large lumps of ice which have not been long melting may easily be 

 below the melting point inside. 



It may be noted that 760 mm. of mercury is not an invariable 



