80 FKAGMENTS OF SCIENCE. 



perature of the vapour may be raised, the rapidity of 

 its vibrations, and consequently its colour, which wholly 

 depends upon that rapidity, remain unchanged. 



The vapour of water, as well as the vapour of silver, 

 has its definite periods of vibration, and these are such 

 as to disqualify the vapour, when acting freely as such, 

 from being raised to a white heat. The oxyhydrogen 

 flame, for example, consists of hot aqueous vapour. It 

 is scarcely visible in the air of this room, and it would 

 be still less visible if we could burn the gas in a clean 

 atmosphere. But the atmosphere, even at the summit 

 of Mont Blanc, is dirty; in London it is more than 

 dirty; and the burning dirt gives to this flame the 

 greater portion of its present light. But the heat of 

 the flame is enormous. Cast iron fuses at a tempera- 

 ture of 2,000 Fahr.; while the temperature of the 

 oxyhydrogen flame is 6,000 Fahr. A piece of platinum 

 is heated to vivid redness, at a distance of two inches 

 beyond the visible termination of the flame. The va- 

 pour which produces incandescence is here absolutely 

 dark. In the flame itself the platinum is raised to daz- 

 zling whiteness, and is even pierced by the flame. When 

 this flame impinges on a piece of lime, we have the 

 dazzling Drummond light. But the light is here due 

 to the fact that when it impinges upon the solid body, 

 the vibrations excited in that body by the flame are of 

 periods different from its own. 



Thus far we have fixed our attention on atoms and 

 molecules in a state of vibration, and surrounded by a 

 medium which accepts their vibrations, and transmits 

 them through space. But suppose the waves generated 

 by one system of molecules to impinge upon another 

 system, how will the waves be affected? Will they be 

 stopped, or will they be permitted to pass? Will they 

 transfer their motion to the molecules on which they 



