370 THE AIR OF TOWNS. 



ordinary circumstances. It is only when a beam of light in a darkened 

 place, a ray of sunlight in a room, a street lamp on a dark night, illu- 

 minate these little particles so that they stand out against a darker 

 background that we see them — the so-called motes dancing in the beam. 

 It is, in fact, these little particles which make the beam of light. With- 

 out the particles the path of the beam would be invisible. The path of 

 light from a luminous body without solid matter to obstruct and reflect 

 it is absolute and unqualified darkness. Here, if I pass a strong beam 

 of light from an electric arc lamp (fig. -23) through the side windows in 

 this wooden box free from dust the beam is cut out where it enters the 

 box and reappears on the other side, where the light emerges. 



We can learn something more from this experiment. The dust is 

 mainly organic; that is, the product, living or dead, of animal and plant 

 life, living germs or dead spores or animal and vegetable refuse matter; 

 for if I now bring a red-hot poker or a Bunsen flame beneath the beam, 

 black smoke appears to rise. The black smoke merely indicates the 

 absence of dust particles where they are burnt up by contact with the 

 source of heat. I will now perform a third instructive experiment to 

 show how little of the dust breathed into our lungs finds its way back 

 into the air; for the air passing out of the lungs cuts a hole in the beam, 

 showing the absence of dust in the breath. These interesting experi- 

 ments were first devised by the late Professor Tyndall. 



I take an ordinary lamp chimney (fig. 24), at the bottom of which 

 a bent tube passes through a cork. By breathing out air from the lungs 

 at the constricted part of the beam, the beam is interrupted from the 

 absence of dust. 



And now let us see how far our theory of fog is capable of illus- 

 tration. 



I have in this large glass vessel (fig. 25) air standing over water. 

 The air is of course saturated with moisture. There is within the 

 vessel a little electric lamp, which will render more evident any 

 change taking place within. If I cool the air, moisture will be depos- 

 ited, but according to our theory it should only appear as mist, if dust 

 particles are present. As the air in the vessel has been standing out 

 of contact with the outside atmosphere for two days, we may assume 

 that the dust now has all subsided and dropped into the water. On 

 cooling the air, we should see no mist. We can cool the air conven- 

 iently and rapidly by making use of the property which air possesses 

 of becoming colder on sudden expansion. I have only then to exhaust 

 the air partially by an air pump by attaching it to this bent tube which 

 passes into the interior to produce the necessary conditions for the 

 formation of mist. Now I have done so, and you observe that no mist 

 appears. I can now, through this second bent tube, draw in a little air 

 laden with dust from the room. I will now cool the air again, and you 

 see at once that a fog appears within the vessel. If I pass in more dust 

 particles, which I am now doing, and punip out the air again, we have 



