90 Introduction to the Study of Science 



peratures. Some are made of clay in the form of pyramids 

 or cones. A pyrometer may have two or more cones in series, 

 each cone made of a different kind of clay so that they melt 

 at different temperatures. The melting temperature of each 

 is known, and that just showing the effect of heat is taken to 

 indicate the temperature. 



Thermometers containing a gas, as hydrogen, are necessary 

 in temperature measurements of extreme accuracy and delicacy. 

 These are usually employed in laboratory investigations. The 

 standard thermometer for the world, however, is a hydrogen 

 thermometer, kept at the International Bureau of Standards, 

 near Paris. 



Mercury freezes at 39 C. What degree on the F. scale? 

 The mercury thermometer is therefore useless for temperatures 

 below this, as in the polar regions and at very high altitudes. 

 Alcohol, which freezes at -130 C. or -202 F., is used for 

 such temperatures. When temperatures below the freezing 

 point of alcohol are to be determined, they are measured with 

 thermometers made of hydrogen gas or of helium, one of the 

 rare gases of our atmosphere. 



It is interesting to note some of the low temperatures obtained 

 in scientific research. Air becomes liquid at a temperature 

 of about -212 C., or -350 F. Hydrogen, when liquefied, 

 produces by its rapid evaporation a temperature of about 

 258 C. Helium is liquefied and by rapid evaporation pro- 

 duces a temperature of 272 C. The lowest possible tempera- 

 ture is 273 C., known in science as absolute zero. The 

 boiling point of liquid air (the boiling point being the tempera- 

 ture at which the liquid is changed into vapor, just as water 

 at 100 C. is converted into vapor or steam) is about 191 C. 

 The boiling point of helium is about 268.5 C. 



Man has learned how to produce and use extremes of tem- 

 perature greater than those found in nature. The lowest 

 temperature obtained is as low, perhaps several degrees lower, 

 than the estimated lowest temperature of the great void among 



