ICE, WATER, AND STEAM 117 



escape, either by breaking the boiler or through some means 

 provided for its escape, as through a safety valve. This 

 power of expansion is utilized by means of the steam engine. 



Steam is a transparent, colorless gas. When steam escapes 

 into the air, there is always a white cloud. This cloud is not 

 steam. The steam quickly cools to a temperature near that 

 of the air, and this is so much below the boiling point that 

 the steam returns to the liquid form. The white cloud is made 

 up of many small drops of liquid water. 



It may be observed that the steam which emerges from a 

 teakettle in which water is boiling is transparent as it passes 

 from the end of the spout. In this transparent area the steam 

 has not yet condensed (fig. 58). 



134. The usual states of matter. In previous work we 

 have seen that the same substance may occur in solid, liquid, 

 or gaseous form. This is noticed more frequently in the case 

 of water than with any other common substance, but it will 

 be worth while, in the following study of water, to remember 

 that many of the things which are true of it are true of other 

 substances also. We need to be clear about the names which 

 are used for the different states of water. The liquid form 

 is most commonly found, and to this form the name "water" 

 is especially applied. It must be remembered that when 

 liquid water evaporates (thus passing into the form of gas) 

 or when it freezes (becoming a solid), it does not cease to be 

 water. For instance, although the water contained in the air 

 is a gas, usually called water vapor, we may speak of it as 

 water ; and when water is a solid we call it ice ; but it is 

 water whatever its form may be. 



135. Distillation. Formerly it was customary for ships to 

 carry with them enough fresh water to last throughout the 

 voyage, but some steamships now prepare their daily sup- 

 ply from the salt ocean water by distillation. When water 

 containing salt or other solid impurity is boiled and thus 

 evaporated, the solid does not evaporate. Since the steam 



