282 FKAGMENTS OF SCIENCE. 



mote-filled air. My idea is that it heats the air and lightens 

 it, without in the same degree lightening the floating mat- 

 ter. The tendency, therefore, is to start a current of clean 

 air through the mote-filled air. Figure the motion of the 

 air all round the wire. Looking at its transverse section 

 we should see the air at the bottom of the wire bending 

 round it right and left in two branch-currents, ascend- 

 ing its sides and turning to fill the^ partial vacuum created 

 above the wire. Now as each new supply of air, filled with 

 its motes, comes in contact with the hot wire, the clean 

 air, as just stated, is first started through the inert motes. 

 They are dragged after it, but there is a fringe of cleansed 

 air in advance of the motes. The two purified fringes of 

 the two branch-currents unite above the wire, and, keep- 

 ing the motes that once belonged to them right and left, 

 they form by their union the dark band observed in the 

 experiment. This process is incessant. Always the mo- 

 ment the mote-filled air touches the wire this distribution 

 is effected, a permanent dark band being thus produced. 

 Could the air and the particles under the wire pass through 

 its mass we should have a vertical current of particles, but 

 no dark band. For here, though the motes would be left 

 behind at starting, they would hotly follow the ascending 

 current and thus abolish the darkness. 



It has been said that when the platinum wire is intensely 

 heated, the floating matter is not only distributed, but de- 

 stroyed. Let this be proved. I stretched a wire about 4 

 inches long through the air of an ordinary glass shade, rest- 

 ing on its stand. Its lower rim rested on cotton-wool, which 

 also surrounded the rim. The wire was raised to a white 

 heat by an electric current. The air expanded, and some 

 of it was forced through the cotton-wool, while, when the 

 current was interrupted and the air within the shade cooled, 

 the expelled air in its return did not carry motes along with 

 it. At the beginning of this experiment the shade was 



