SPONTANEOUS GENERATION 327 



with the exception of an aperture in a shutter through 

 which a sunbeam enters and crosses the room. The float- 

 ing dust reveals the track of the light. Let a lens be 

 placed in the aperture to condense the beam. Its parallel 

 rays are now converged to a cone, at the apex of which 

 the dust is raised to almost unbroken whiteness by the 

 intensity of its illumination. Defended from all glare, 

 the eye is peculiarly sensitive to this scattered light. The 

 floating dust of London rooms is organic, and may be 

 burned without leaving visible residue. The action of a 

 spirit-lamp flame upon the floating matter has been else- 

 where thus described: 



In a cylindrical beam which strongly illuminated the 

 dust of our laboratory, I placed an ignited spirit-lamp. 

 Mingling with the flame, and round its rim, were seen 

 curious wreaths of darkness resembling an intensely black 

 smoke. On placing the flame at some distance below the 

 beam, the same dark masses stormed upward. They were 

 blacker than the blackest smoke ever seen issuing from the 

 funnel of a steamer; and their resemblance to smoke was so 

 perfect as to prompt the conclusion that the apparently pure 

 flame of the alcohol-lamp required but a beam of sufficient 

 intensity to reveal its clouds of liberated carbon. 



But is the blackness smoke? This question presented 

 itself in a moment, and was thus answered: A red-hot poker, 

 was placed underneath the beam; from it the black wreaths 

 also ascended. A large hydrogen flame, which emits no 

 smoke, was next employed, and it also produced with aug- 

 mented copiousness those whirling masses of darkness. 

 Smoke being out of the question, what is the blackness ? 

 It is 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 



