14 THE PRODUCTION AND USE OF LIGHT 



ice always melts at a certain temperature. For pure water 

 this temperature is called the FKEEZING POINT OF WATEE. 



2. It has also been known that at the average atmospheric 

 pressure at sea level water always boils at a certain tem- 

 perature. The temperature of the steam arising from water 

 boiling under the atmospheric pressure which exists at the sea 

 level is, therefore, called the BOILING POINT OF WATEE. 



3. About two hundred years ago Fahrenheit found that a 

 mixture consisting of 1 part common salt and 2J^ parts snow 

 or crushed ice always melted at a certain temperature consid- 

 erably below the temperature of freezing water. This third 

 temperature is known as the MELTING POINT OF A MEXTUEE OF 



SALT AND ICE. 



There are a great many other fixed points of temperature in 

 nature. For example, pure iron always melts at exactly the 

 same temperature; so does lead, and so does zinc. Both pure 

 mercury and pure alcohol have certain temperatures at which 

 they boil and others at which they freeze. When in good health 

 the human blood always has the same temperature. But in 

 marking the common thermometer the three temperatures 

 first mentioned, namely, the freezing point of water, the boiling 

 point of water, and the melting point of a mixture of ice and 

 salt, are the only temperatures which we use. 



17. Fahrenheit's Thermometer. In making the ther- 

 mometer which bears his name, Fahrenheit marked the posi- 

 tion of the mercury "Q" when the thermometer was cooled 

 down to the temperature of the mixture of ice and salt. It 

 is believed that he was induced to call this temperature 

 partly because it was the unusually low temperature reached 

 by the weather in Holland in the winter of 1709. He called 

 the temperature of freezing water "32" and the temperature 

 of boiling water "212." 



18. Centigrade Thermometer. Much later Celsius made 

 his so-called CENTIGEADE THEEMOMETEE. On this ther- 

 mometer the freezing point of water is marked as "0" and 

 the boiling point of water as "100." 



