A HARD-WORKING DIET. 335 



behaving, but where it is not convenient to do this, 

 perhaps it is still possible to convey some ideas about 

 them, if not very complete and exact. 

 To those who feel, 



" A fire's a good companionable friend, 

 Who meets your face with welcome glad," 



and love to loiter in the gloaming and gaze into it, 

 OXYGEN is a perfectly familiar though unseen friend. Oxygen. 



The old adage " seeing is believing " is taken by 

 some as equivalent to "do not believe what you 

 cannot see." But we believe in many things we 

 cannot see, when we can see what they do. We cannot 

 see the wind, but we are constrained to believe in it 

 if it brings a chimney pot down through the roof. 

 When, sheltered by a window, we watch the boughs 

 swaying and the clouds scudding along in fantastic 

 forms, or perhaps smile at undignified chases after 

 runaway hats, we do not hesitate to say "see what 

 a wind there is ! " 



Oxygen is a perfectly familiar gas, because any one 

 watching a fire is seeing one of the things the unseen 

 Oxygen is always somewhere doing. One-fifth of the 

 air is free Oxygen. Every one knows that a fire or a 

 lamp will not burn without air, though they may not 

 know that it is only the Oxygen of the air that is con- 

 cerned in the burning. The rest of the air has nothing 

 to do with it so far as we know. In pure Oxygen, 

 which can be obtained in several ways, burning is 

 much more brilliant. The burning of a watchspring 

 in Oxygen is a sight young and old enjoy, for as a 

 professor at the Royal Institution used to say to his 

 audience, one is never tired of seeing it. 



