THE EFFECTS OF HEAT. 



239 



Is a boiling 

 temperature 

 reiMisite for 

 tile production 

 9\ iieam ? 



Is vapor al- 

 ways present 



Fig. 207, 



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 multitude of these reflecting points, there- 

 fore, make the space through which ihey are difl'used appear like a cloudy 

 body, more or less white, accordmg to their abundance. 



The surface of any watery liquid, whose temperature is 

 about 20° warmer than any superincumbent au", rapidly gives 

 off true steam. It is not necessary, therefore, for the produc- 

 tion of steam that water should be raised to the boihng tem- 

 perature. 



549. Air without vapor (tlieoretically called 

 dry air) is not known to exist in nature, and is 

 probably not producible by art. 



550. Liquids in passing into vapors occupy 

 a much greater space than the substances from 

 which they are produced. Water, in pass- 

 ing from its point of greatest density into 



steam, expands to nearly 1700 times its volume. 



Fig. 207 represents the comparative 

 volume of water and steam. 



551. Vapors are 



of all degrees of 



density. The va- 

 por of water may be as thin as 

 air, or almost as dense as water. 



The opinion formerly prevailed that va- 

 pors could not exist by themselves as 

 Bucli, but that they were dissolved in the 

 air in the same way as salt is dissolved in 

 water. The fallacy of this idea is proved 

 by the fact that evaporation goes on more 

 rapidly in a vacuum, where no substance whatever is present, than in the 

 air. 



552. Evaporation takes place from the sur- 

 faces of bodies only, and is influenced in a 

 great degree by the temperature, dryness, still- 

 ness, and density of the atmosphere. 



The effect of temperature in promoting evaporation may bo 

 perature influ- illustrated by placing an equal quantity of water in two sau- 

 cers, one of which is placed in a warm and dry, and the other 

 in a cold and damp, situation. The former will be quite dry 

 before the latter has suffered an appreciable diminution. 



in air f 



What is the 

 relative space 

 occupied by 

 liquids and va- 

 pors? 



Is the density 

 of vapors uni- 

 form? 



What circum- 

 St;inces iiirtu- 

 ence evapora- 

 tion ? 



ence evapora- 

 tion? 



