OBJECTS.] INTRODUCTORY. 57 



allowed to stand even in a cool room or in the open 

 air, you know that it sooner or later disappears. Wet 

 clothes hung on a line soon dry that is to say, the 

 water clinging to them disappears or evaporates. 

 The disappearance of the water under these circum- 

 stances results from the property just mentioned. In 

 fact, it becomes gaseous water of the density appro- 

 priate to the temperature, and as such mixes with the 

 air as any other gas would do. And as the sea, 

 lakes, and rivers, are constantly giving off gaseous 

 water into the air in proportion to the temperature, 

 it is not wonderful that the atmosphere always contains 

 gaseous water. 



Air is said to be moist when the weight oi water in 

 a given quantity, say 100 cubic inches, is as much, or 

 nearly as much, as can exist in the state of gas at the 

 temperature. Under these circumstances, if the tem- 

 perature is lowered even a very little, some of the 

 gaseous water is converted into liquid water. We 

 see this in hot moist weather, when the outside of a 

 tumbler of fresh drawn cold spring water immediately 

 becomes bedewed. The gaseous water in immediate 

 contact with the tumbler, in fact, is cooled down below 

 the point at which it can all exist as gas, and the 

 superfluity is deposited as dew. In such days wet 

 clothes do not dry well, because there is, already, 

 nearly as much gaseous water in the atmosphere as 

 the amount of heat marked by the thermometer can 

 maintain in that state. 



39. When Hot Water is Cooled, it Con- 

 tracts to begin with, but after a time Expands. 



We have now seen what a wonderful change is 



