ON DUST AND DISEASE. 



I. 



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 ; 



