280 FRAGMENTS OF SCIENCE. 



In a cylindrical beam, which strongly illuminated the 

 dust of the laboratory, was 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 lowering the flame below the beam the same 

 dark masses stormed upward. They were at times blacker 

 than the blackest smoke that I have ever seen issuing from 

 the funnel of a steamer ; and their resemblance to smoke 

 was so perfect as to lead the most practised observer to 

 conclude 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. A red-hot poker was placed under- 

 neath the beam, and from it the black wreaths also as- 

 cended. A large hydrogen-flame was next employed, and 

 it produced those whirling 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 particles, 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, un- 

 seen, 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 illumina- 

 tion. 



But here a rather perplexing difficulty meets us. It is 

 not necessary to burn the particles to produce a stream of 



