perfectly polarized light. The cloud could be utterly 

 quenched by a transparent Nicol's prism, and the tube 

 containing it reduced to optical emptiness. 



The particles floating in the air of London being thus 

 proved to be organic, I sought to burn them up at the 

 focus of a concave reflector. One of the powerfully 

 convergent mirrors employed in my experiments on 

 combustion by dark rays was here made use of, but I 

 failed in the attempt. Doubtless the floating particles 

 are in part transparent to radiant heat, and are so far 

 incombustible by such heat. Their rapid motion through 

 the focus also aids their escape. They do not linger 

 there sufficiently long to be consumed. A flame it was 

 evident would burn them up, but I thought the presence 

 of the flame would mask its own action among the par- 

 ticles. 



In a cylindrical beam, which powerfully illuminated 

 the dust of the laboratory, was placed an ignited spirit- 

 lamp. Mingling with the flame, and round its rim, were 

 seen wreaths of darkness resembling an intensely black 

 smoke. On lowering the flame below the beam the 

 same dark masses stormed upwards. They were at times 

 blacker than the blackest smoke that I have ever seen 

 issuing from the funnel of a steamer, and their resem- 

 blance to smoke was so perfect as to lead the most prac- 

 ticed observer to conclude that the apparently pure 

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

 cient intensity to reveal its clouds of liberated carbon. 



But is the blackness smoke ? The question presented 

 itself in a moment. A red-hot poker was placed under- 

 neath the beam, and from it the black wreaths also 

 ascended. A large hydrogen flame was next employed, 

 and it produced those whirling masses of darkness far 



