102 SXPEKIMEKTAL. GENERAL SCIENCE 



boiling points. On a eold day, ioe will sublimate, and a light 

 snow fall may thus disappear without making the ground wet. 

 Frost frequently sublimates instead of melting. Other sub- 

 stances are known which, owing to their readiness to unite 

 with oxygen, cannot be turned to gases under ordinary condi- 

 tions. When heated beyond a certain point they do not even 

 become liquid but form gaseous compounds with the oxygen 

 in the air. Carbon and phosphorus are elements of this kind. 

 90. Boiling. If the temperature of an evaporating liquid 

 be increased sufficiently, the surface will not provide enough 

 space for the escape of all the vapor produced. In conse- 

 quence, bubbles of the vapor begin to form within the body 

 of the liquid itself at the point where the heat is being applied. 

 When the temperature is sufficiently high to cause these bub- 

 bles to rise to the surface and escape into the air, we speak of 

 the process as boiling. For some time before boiling begins, 

 the bubbles of vapor may rise into the cooler parts of the liquid 

 and there be condensed to liquid again. This condition is 

 called simmering. In a liquid exposed to the air, boiling can 

 occur only when the average speed of its molecules exceeds 

 the speed of the molecules of the air, otherwise they will be 

 knocked back into the liquid again by collision with the air 

 molecules. This explains why compressing the air over a 

 boiling liquid increases the boiling point. The molecules of 

 air pushed closer together and moving at higher speeds make 

 the escape of the molecules of the liquid more difficult (61). 

 If the pressure be reduced, however, the jnolecules escape at 

 a much lower temperature. Water evaporates into a vacuum 

 almost instantly. If the air did not hinder evaporation, we 

 would be living in an atmosphere saturated with moisture. 

 Though the pressure of a gas over a liquid may retard the 

 evaporation into it, it does not permanently prevent it. As 

 much water will eventually evaporate into an air-filled space 

 as would evaporate into it if the air were not there. 



