284 FRAGMENTS OF SCIENCE. 



4. The experiment was repeated, with the difference 

 that the air was sent more slowly through the red-hot tube. 

 The floating matter was totally destroyed. 



5. The platinum tube was now lowered until it bordered 

 upon a visible red heat. The air sent through it still more 

 slowly than in the last experiment carried with it a cloud 

 of floating matter. 



If, then, the suspended matter is destroyed by a bright- 

 red heat, much more is it destroyed by a flame, whose tem- 

 perature is vastly higher than any here employed. So that 

 the blackness introduced into a luminous beam where a 

 flame is placed beneath it is due, as stated, to the destruc- 

 tion of the suspended matter. At a dull-red heat, how- 

 ever, and still more when only on the verge of redness, the 

 platinum tube permitted the motes to pass freely. In the 

 latter case the temperature was 800° or 900° Fahrenheit. 

 This was unable to destroy the suspended matter ; much 

 less, therefore, would a platinum -wire heated to 212° be 

 competent to do so. Such a wire can only distribute the 

 matter, not destroy it. 



The floating dust is revealed by intense local illumina- 

 tion. It is seen by contrast with the adjacent illuminated 

 space ; the brighter the illumination the more sensible is 

 the difference. Now, the beam employed in the foregoing 

 experiments is not of the same brightness throughout its 

 entire transverse section. Pass a white switch, or an ivory 

 paper-cutter, rapidly across the beam, the impression of its 

 section will linger on the retina. The section seems to float 

 for a moment in the air as a luminous circle, with a rim much 

 brighter than its central portion. The core of the beam is 

 thus seen to be enclosed by an intensely-luminous sheath. 

 An effect complementary to this is observed when the beam 

 is intersected by the dark band from the platinum wire. 

 The brighter the illumination the greater must be the rela- 

 tive darkness consequent on the withdrawal of the light. 



