ON DUST AND DISEASE. 135 



To study this effect a platinum wire was stretched 

 across the beam, the two ends of the wire being con- 

 nected 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 long before it reached the heat of ignition, a flat 

 stream of air rose from it, which when looked at edge- 

 ways appeared darker and sharper than one of the 

 blackest lines of Fraunhofer in the purified 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 the inert particles, dragging 

 them after it right and left, but forming between them 

 an impassable black partition. This elementary ex- 

 periment enables us to render an account of the dark 

 currents produced by bodies at a temperature below 

 that of combustion. 



But when the platinum wire is intensely heated, 

 the floating matter is not only displaced, but destroyed. 

 I stretched a wire about 4 inches long througli the air 

 of an ordinary glass shade resting on cotton-wool, 

 which also surrounded the rim. The wire being raised 

 to a white heat by an electric current, the air expanded, 

 and some of it was forced through the cotton-wool. 

 When the current was interrupted, and the air within 

 the shade cooled, the returning air did not carry motes 

 along with it, being filtered by the wool. At the 

 beginning of this experiment the shade was charged 

 with floating matter ; at the end of half an hour it. 

 was optically empty. 



