131 



V. 



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.' 



In my researches on the decomposition of vapours 

 by light, I was compelled to remove these ' atomes ' 

 and this dust. It was essential that the space con- 

 taining the vapours should embrace no visible thing 

 that no substance capable of scattering light in the 

 slightest sensible degree should, at the outset of an 

 experiment, be found in the wide ' experimental tube ' 

 in which the vapour was enclosed. 



For a long time I was troubled by the appearance 

 there of floating matter, which, though invisible in 

 diffuse daylight, was at once revealed by a powerfully 

 condensed beam. Two U-tubes were placed in suc- 

 cession in the path of the air, before it entered the 

 liquid whose vapour was to be carried into the experi- 

 mental tube. One of the U-tubes contained fragments 

 of marble wetted with a strong solution of caustic potash ; 

 the other, fragments of glass wetted with concentrated 



1 A discourse delivered before the Royal Institution of Great 

 Britain, Jauary 21, 1870. 



x 2 



