XI. 



ON DUST AND DISEASE. 



Experiments on Dusty Air. 



Solar light in passing through a dark room reveals its 

 track by illuminating the dust floating in the air. " The 

 sun," says Daniel Culverwell, " discovers atomes, though 

 they be invisible by candle-light, and makes them dance 

 naked in his beams." 1 



In my researches on the decomposition of vapors by 

 light, I was compelled to remove these " atomes " and this 

 dust. It was essential that the space containing the vapors 

 should embrace no visible thing ; that no substance capable 

 of scattering the light in the slightest sensible degree 

 should, at the outset of an experiment, be found in the " ex- 

 perimental tube " traversed by the luminous beam. 



For a long time I was troubled by the appearance there 

 of floating dust, which though invisible in diffuse daylight 

 was at once revealed by a powerfully-condensed beam. Two 

 tubes were placed in succession in the path of the air : the 

 one containing fragments of glass wetted with concentrated 

 sulphuric acid ; the other, fragments of marble wetted with 

 a strong solution of caustic potash. To my astonishment 

 the dust passed through both. The air of the Royal Insti- 

 tution sent through these tubes at a rate sufficiently slow 



1 On a day of transient shadows there is something almost magical in 

 the rise and dissolution of the luminous beams among the scaffolding 

 poles of the Royal Albert Hall. 



