188 



THE DEPOSITION OF SMOKE AND DUST. 



As an upward stream of hot air is capable of displacing 

 the particles, so can thoy be displaced by introdacing into 

 their midst a gas- so prepared that in it suspended particles 

 do not exist. O-xygcn and ordinary coal gas produce the 

 black space equally well as a stream of hot air. 



It is well known that if the- air in a room bo left to itself 

 for a time, and no currents or draughts of air bo allowed to 

 enter, the dust in the air of that room will quietly settle 

 down on the articles of furniture and even upon the walls. 

 So that could we, without disturbing in any way tlie con- 

 fined air, shoot a beam of light across the room, instead of 

 the track of that beam being rendered visible by the float- 

 ing dust, the beam would only be seen where it fell on the 

 opposite side. 



Professor Tyndall, while engaged in his researches on 

 Disease Germs, was by means of the luminous beam able to 

 prove that towards the end of an oxpiratiom of air from the 

 lungs, the air expired was entirely free from suspended 

 matter, the lungs acting as filter's holding back all particles 

 of dust inhaled. By breathing through a tube across the 

 track of a beam, he noticed that tlie beam appeared as 

 pierced by an intensely black liole, which was formed by the 

 deeper air from his 1 ungs. 



Absolute freedom from dust is not only obtained by means 

 already mentioned, but it has been demonstrated experi- 

 mentally that any hot body is surrounded by a film of air 

 which is absolutely free from dust, and this hot body pro- 

 duces an ascending dust- free stream of air.. 



The displacement of the dust particles is most probably 

 due to the same cause that produced a similar effect in 

 the beam,— namely, the rarefaction of the air immediately 

 surrounding the body, the dust particles preferring to lag 

 behind and enjoy the close company of their follows in 



