4 THE FLOATING-MATTER OF THE AIR. 



it the black wreaths also ascended. A large hydrogen 

 flame was next employed, and it produced those whirl- 

 ing masses of darkness, far 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 par- 

 ticles, and substituted 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 visible. 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. 



It is not, however, necessary to burn the particles 

 to produce a stream of darkness. Without actual 

 combustion, currents may be generated which shall 

 displace the floating matter, and appear dark amid the 

 surrounding brightness. I noticed this efiect 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. 



* In none of the public rooms of the United States where I had 

 the honour to lecture was this experiment made. The organic 

 dust was too scanty. Certain rooms in England — the Brighton 

 Pavilion, for example— also lack the necessary conditions. 



