23$ ( 2 4 ) 



more copiously than either the spirit-flame or poker. 

 Smoke was, therefore, out of the question. 



What, then, was the blackness ? It was simply that 

 of stellar space ; that is to say, blackness resulting from 

 the absence from the track of the beam of all matter 

 competent to scatter its light. When the flame was 

 placed below the beam the floating matter was destroyed 

 in situ ; and the air, freed from this matter, rose into the 

 beam, jostled aside the illuminated particles and substi- 

 tuted for their light the darkness due to its own perfect 

 transparency. Nothing could more forcibly illustrate 

 the invisibility of the agent which renders all things vis- 

 ible. The beam crossed, unseen, the black chasm formed 

 by the transparent air, while at both sides of the gap 

 the thick-strewn particles shone out like a luminous solid 

 under the powerful illumination. 



But here a difficulty meets us. It is not necessary to 

 burn the particles to produce a stream of 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 below 

 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 con- 

 nected with the two poles of a voltaic battery. To reg- 

 ulate 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 



