THE CHEMISTRY OF THE ATMOSPHERE. 107 



are not wholly dependent upon the air for their oxygen. 

 Plants that grow under water continually discharge oxygen 

 from their leaves, just as is done from leaves in the air; and 

 there is the same chemical commerce between animals and 

 plants under water that there is in the air, though it is not 

 so extensive. The fishes and other animals give carbonic 

 anhydride to the plants, and take from them oxygen in re- 

 turn. The oxygen can often be seen gathered in globules 

 on the surface of water-plants, waiting to be dissolved by 

 the water. A suitable regard to this exchange between 

 plants and animals under water suggests the presence of 

 water-plants in an aquarium as a necessary part of the ap- 

 paratus, they getting from the lungs of the animals carbon 

 for their growth, and breathing back to them from the pores 

 of their leaves oxygen in return. 



137. "Water in the Air. As water dissolves air, so air dis- 

 solves water when the latter is in a gaseous state. There 

 is always water in the atmosphere, even when it seems to 

 be perfectly dry. It is invisible because it is in its vapor- 

 ous form, and so its particles are intimately mingled with 

 the gaseous particles of the air. This solution of water in 

 air is like the solution of some solids in water. If alum, for 

 example, be dissolved in water, it disappears, and so does 

 the water dissolved in air. And as warm water will dis- 

 solve more alum than cold water, so will warm air dissolve 

 more water than cold air. There is therefore more water 

 in the air in summer than in winter. Sometimes there is as 

 much as one gallon of water to every sixty gallons of the 

 air. The analogy can be traced farther. If warm air be 

 chilled in any way, it can not hold as much water in solu- 

 tion, and some of it, therefore, is separated from the air that 

 is, taken out from that intimate union which constitutes so- 

 lution. This separated part of the water may appear as fog 

 or cloud, or be deposited as dew or frost, or fall as rain, 



