( 2 5) 239 



before it reached the heat of ignition, a flat stream of 

 air rose from it, which when looked at edgeways ap- 

 peared darker and sharper than one of the blackest 

 lines of Fraunhofer in the solar spectrum. Right and 

 left of this dark vertical band the floating matter rose 

 upwards, bounding definitely the non-luminous stream 

 of air. What is the explanation ? Simply this. The 

 hot wire rarefied the air in contact with it, but it did not 

 equally lighten the .floating matter. The convection 

 current of pure air therefore passed upwards among ihe 

 particles, dragging them after it right and left, but form- 

 ing between them an impassable black partition. In 

 this way we render an account of the dark currents pro- 

 duced by bodies at a temperature below that of combus- 

 tion. 



Oxygen, hydrogen, nitrogen, carbonic acid, so pre- 

 pared as to exclude all floating particles, produce the 

 darkness when poured or blown into the beam. Coal- 

 gas does the same. An ordinary glass shade placed in 

 the air with its mouth downwards permits the track of 

 the beam to be seen crossing it. Let coal-gas or hydro- 

 gen enter the shade by a tube reaching to its top, the 

 gas gradually fills the shade from the top downwards. 

 As soon as it occupies the space crossed by the beam, 

 the luminous track is instantly abolished. Lifting the 

 shade so as to bring the common boundary of gas and 

 air above the beam, the track flashes forth. After the 

 shade is full, if it be inverted, the gas passes upwards 

 like a black smoke among the illuminated particles. 



The air of our London rooms is loaded with this or- 

 ganic dust, nor is the country air free from its pollution. 

 However ordinary daylight may permit it to disguise 

 itself, a sufficiently powerful beam causes the air in 



