22 ELEMENTS OF GENERAL SCIENCE 



of a gas (water vapor) it is invisible, just as is true of the 

 remainder of the atmosphere. 



22. Condensation. Not only does water readily disappear 

 into the air but it sometimes makes its appearance more or 

 less mysteriously at a point where previously there has been 

 no water. Thus it is a common observation that if a glass 

 pitcher which has been carefully dried on the outside be filled 

 with ice water, the outside of the pitcher may soon become 

 moist. Sometimes one is inclined to account for the phe- 

 nomenon by supposing that by some means water has got 

 out of the pitcher to the outside. A little reflection shows 

 that this cannot be the explanation, since the same thing will 

 occur to a cold piece of metal if it is brought into the room; 

 also those who wear eyeglasses are annoyed by the moisture 

 which collects in their glasses when they go from cold air 

 out of doors into warm living rooms. Indeed, if the ice water 

 were inclosed within a tightly stoppered bottle instead of 

 being exposed to the air in an open pitcher, the collection of 

 moisture would be in no wise interfered with. Out of doors, 

 at night, water commonly collects on cold objects, and some- 

 times the air itself becomes filled with a fog consisting of 

 minute floating particles of water. All of these phenomena 

 may occur quite independently of any visible source of water, 

 such as a pond or lake. 



We must therefore conclude that the water which appears 

 upon cold objects probably comes from the air, just as the 

 water which evaporates goes into the air. We may conclude 

 also, since it appears, or condenses, only upon objects rela- 

 tively cooler than the air, that the temperature has something 

 to do with it. But these conclusions, even if verified by other 

 experiments, do not solve all the problems connected with the 

 deposition of water on the pitcher. Why is not water always 

 condensed upon the outside of vessels containing ice water? 

 Is it because there is at times little water in the air or no 

 water at all or because of uniform temperatures ? 



