SPONTANEOUS GENEEATION. 297 



— ' Stickstoffsplittem,' as Cohn contemptuously calls 

 them — is followed by the sprouting of infusorial life. 



Let us now return to London and fix our attention 

 on the dust of its air. Suppose a room in which the 

 housemaid has just finished her work to be completely 

 closed, with the exception of an aperture in a shutter 

 through which a sunbeam enters and crosses the room. 

 The floating 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 elsewhere thus descriJDed :- — 



In a cylindrical beam which strongly illnminated the 

 dust of our laboratory, I placed an ignited spmt 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 upwards. They were 

 blacker than the blackest smoke ever seen issuing from the 

 fvmnel 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 

 suiEcient intensity to reveal its clouds of liberated carbon. 



But is the blackness smoke 1 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 augmented copiousness those whirling masses of dark- 

 ness. Smoke being out of the question, what is the black- 



