248 Domestic Science 



inch. A third screw is to be dropped into this jar, 

 which is then securely stoppered. Examine the appear- 

 ance of this screw at the same times as the others are 

 examined. 



From the observations made in the above experiment 

 it will probably be inferred that neither pure water, 

 free from dissolved gases, nor dry air, can alone cause 

 the formation of rust. Exposure to the combined 

 effects of moisture and air, however, readily brings 

 about this change. The rust is of somewhat variable 

 composition, being a mixture of the brown oxide of 

 iron, called "ferric oxide", with a compound of this 

 oxide with the elements of water, called " ferric 

 hydroxide ". It seems probable that the carbon dioxide 

 of the air plays a part in the rusting, since the addition 

 of a little solution of caustic potash to the water will 

 prevent the formation of the layer of rust. 



168. The gases of the air dissolved in water are of 

 considerable importance to the maintenance of life. 

 We have already noted the part played by the dissolved 

 carbon dioxide in the vital processes of water-plants. 

 The dissolved oxygen has an equally important function 

 in that aquatic animals are dependent upon it for their 

 supply of this gas, which is as necessary for their 

 existence as for that of ourselves and all other land 

 animals. The gills of a fish are composed of delicate 

 membranes through which the air dissolved in the 

 water is able to pass, the fish using the oxygen and 

 returning the other gases and the carbon dioxide pro- 

 duced by the oxidation of its food to the water in 

 which it lives. If placed in recently boiled and cooled 

 water, a fish rises to the top of the water in a gasping 

 condition and speedily dies from suffocation if it be not 

 removed into some ordinary water. Again, if a number 



