86 WATER 



from the spout of a tea-kettle, it is invisible until it con- 

 denses to fine drops, some distance away. The fine drops 

 are liquid water, not steam. Clouds are made of similar 

 droplets : ' ' fog, ' ' not of steam. 



While water freezes at a definite temperature (0 C. or 

 32 F.), it is changed into steam at any temperature. 

 Even ice and snow pass directly into steam (evaporate) 

 on a cold winter's day, without melting. 



Like air and other gases, steam has pressure. At the 

 ordinary temperature the pressure of the steam given off 

 by water is small (cf. Appendix, Table VIII) ; but as the 

 temperature of the water rises, its steam has a greater and 

 greater pressure, until at 100 C., or 212 F., the steam has 

 the same pressure as the air (760 mm.). The steam then 

 sweeps the air completely out of the vessel in which the 

 water is being heated. We say that the water is boiling, 

 and we caU 100 C., or 212 F., the boiling point of water 

 (of. 62). 



90. The Boiling Point Changes with Pressure. If 



water is boiled in a closed vessel, more and more steam is 

 packed into the space above the liquid water, and the 

 pressure of the steam increases accordingly. Under the 

 increased pressure of the steam, the water now boils 

 above 100 C. When the steam has twice the atmos- 

 pheric pressure (2X760 mm.), water boils at 121 C. In a 

 locomotive boiler producing steam at 13 "atmospheres" 

 pressure (191 pounds to the square inch) water boils at 

 192 C., or 378 F. When steam at high pressure is allowed 

 to escape, it expands greatly. This is the source of 

 motion in a steam engine (cf. 24). 



