200 SCIENCE OF COMMON THINGS. 



Vapor of water always exists in air. White appearance of steam. 



1336 Does air ever exist without steam or vapor of water ? 



Air without steam (theoretically called dry air) is 

 not ~known to exist in nature, and is probably not pro- 

 ducible by art. 



1337* Is the visible matter, popularly called steam, really true steam ? 



By no means, and should be carefully distinguished 

 from steam proper, or the aeriform state of water. The 

 cloud or smoke-like matter alluded to is really not an 

 air or vapor at all, but a dust-like cloud of minute 

 bodies of liquid water, wafted by a current either of 

 true steam, or, more frequently, of mere moist air. 



1338 Is it necessary to the production of steam that water should be 

 raised to the boiling temperature f 



It is not / the surface of any watery liquid, about 20 

 warmer than any superincumbent air (however warm 

 or cold that may be), rapidly gives off true steam, which 

 is invisible, but which no sooner mixes with colder air 

 than it is recondensed into water, and assumes the 

 forms of minute globules. 



1339 What causes the visible white appearance of condensed steam f 



The myriads of minute globules of water into which 

 the steam is condensed are separately invisible to the 

 naked eye, but each, nevertheless, reflects a minute ray 

 of white light. The multitudes of these reflecting 

 points, therefore, make the space through which they 

 are diffused appear like a cloudy body, more or less 

 white, according to their abundance. 



1330 In what manner is the production of steam in boiling water first 

 manifested ? 



, When steam begins to be produced, as in the process 

 of making water boil, and the heat overcomes the 

 atmospheric pressure on the surface, small bubbles are 

 formed, adhering slightly to the sides of the vessel. 



1331 In what parts of the boiler will its development be most conspicu- 

 ous? 



The bubbles are formed most rapidly at those points 

 against which the flame is most strongly directed. 



1333 How much lighter is steam than water f 



