II. 



On Haze and Dust. 



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." 



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 " experimental 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 dust : 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 it passed through 

 both. The air of the Royal Institution, sent through 

 these tubes at a rate sufficiently slow to dry it and to re- 

 move its carbonic acid, carried into the experimental 

 tube a considerable amount of mechanically-suspended 

 matter, which was illuminated when the beam passed 



