EVAPORATION 41 



purities in this way is called distilled water, and the process 

 is called distillation (Fig. 23). By this method, the salt water 

 of the ocean may be separated into pure drinking water and 

 salt, and many of the large ocean liners distill from the briny 

 deep all the drinking water used on their ocean voyages. 



Commercially, distillation is a very important process. Tur- 

 pentine, for example, is made by distilling the sap of pine 

 trees. Incisions are cut in the bark of the long-leaf pine trees, 

 and these serve as channels for the escape of crude rosin. This 

 crude liquid is collected in barrels and taken to a distillery, 

 where it is distilled into turpentine and rosin. The turpentine 

 is the product which passes off as vapor, and the rosin is the 

 mass left in the boiler after the distillation of the turpentine. 



Evaporation. If a stopper is removed from a cologne 

 bottle, the contents of the bottle slowly evaporates; if a dish 

 of water is placed out of doors on a hot day, evaporation oc- 

 curs very rapidly. The liquids which have disappeared from 

 the bottle and the dish have passed into the surrounding air 

 in the form of vapor. Water passes into vapor only with the 

 addition of heat ; now the heat necessary for the evaporation 

 of the cologne and water was taken from the air, leaving it 

 slightly cooler. If wet hands are not dried with a towel, but 

 are left to dry by evaporation, heat is taken from the hand in 

 the process, leaving a sensation of coolness. Damp clothing 

 should never be worn, because the moisture in it tends to 

 evaporate at the expense of the bodily heat, and this undue 

 loss of heat from the body produces chills. After a bath the 

 body should be well rubbed, otherwise evaporation occurs at 

 the expense of heat which the body cannot ordinarily afford 

 to lose. 



Evaporation is a slow process occurring at all times; it is 

 hastened during the summer, because of the large amount of 

 heat present in the atmosphere. Many large cities make use 



