DUST AND DISEASE. 281 



darkness. Without actual combustion, currents may be 

 generated which shall exclude the floating matter, and 

 therefore appear dark amid the surrounding brightness. I 

 noticed this effect first on placing a red-hot copper ball be-- 

 low the beam, and permitting it to remain there until its 

 temperature had fallen below that of boiling water. The 

 dark currents, though much enfeebled, were still produced. 

 They may also be produced by a flask filled with hot 

 water. 



To study this effect a platinum wire was stretched 

 across the beam, the two ends of the wire being connected 

 with the two poles of a voltaic battery. To regulate the 

 strength of the current a rheostat was placed in the circuit. 

 Beginning with a feeble current the temperature of the 

 wire was gradually augmented ; but, before it reached the 

 heat of ignition, a flat stream of air rose from it, which 

 when looked at edgeways appeared darker and sharper than 

 one of the blackest lines of Fraunhofer in the solar spec 

 trum. Right and left of this dark vertical band the float 

 ing matter rose upward, bounding definitely the non-lumi 

 nous 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 con 

 vection current of pure air therefore passed upward among 

 the inert particles, dragging them after it right and left, but 

 forming between them an impassable black partition. This 

 elementary experiment enables us to render an account of 

 the dark currents produced by bodies at a temperature be 

 low that of combustion. 



When the wire is white hot, it sends up a band of in 

 tense darkness. This, I say, is due to the destruction of 

 the floating matter. But even when its temperature does 

 not exceed that of boiling water, the wire produces a dark 

 ascending current. This, I say, is due to the distribution 

 of the floating matter. Imagine the wire clasped by the 



