42 



A YEAR IN SCIENCE 



is called distilled water, and the process is called distilla- 

 tion. By this method, the salt water of the ocean may be 

 separated into pure drinking water and salt. Distilled 

 water is used by chemists in their work ; it is used in the 

 manufacture of artificial ice and also for drinking water. 



Turpentine is made by distilling the sap of pine 

 trees. The sap is collected and heated. Turpentine 

 passes off as a steam, and rosin is the mass left in the 

 boiler. 



Effect of change of pressure on boiling point. Water 

 is placed in a flask and heated to about 70 C. A ther- 

 mometer is then put in the flask; the whole apparatus 

 is placed under the receiver of an air pump, and the 



Fig. 21. If part of the air is withdrawn by means of an air 

 pump from around the flask, water in the flask will boil at a tem- 

 perature far below 100 C. 



air is exhausted. When the pressure is sufficiently 

 diminished the water boils violently at a temperature 

 far below the usual boiling point. 



