DUST AND DISEASE. 285 



Hence the cross-section of the sheath surrounds the dark 

 band as a darker ring. 



Oxygen, hydrogen, nitrogen, carbonic acid, so prepared 

 as to exclude all floating particles, produce the darkness 

 when poured or blown into the beam. Coal-gas does the 

 same. An ordinary glass shade placed in the air with its 

 mouth downward permits the track of the beam to be seen 

 crossing it. Let coal-gas or hydrogen enter the shade by a 

 tube reaching to its top, the gas gradually fills the shade 

 from the top downward. As soon as it occupies the space 

 crossed by the beam, the luminous track is instantly abol- 

 ished. Lifting the shade so as to bring the common bound- 

 ary of gas and air above the beam, the track flashes forth. 

 After the shade is full, if it be inverted, the gas passes up- 

 ward like a black smoke among the illuminated particles. 



The air of our London rooms is loaded with this organic 

 dust, nor is the country air free from its presence. How r ever 

 ordinary daylight may permit it to disguise itself, a suffi- 

 ciently powerful beam causes dust suspended in air to ap- 

 pear almost as a semi-solid. Nobody could, in the first 

 instance, without repugnance, place the mouth at the illu- 

 minated focus of the electric beam and inhale the thickly- 

 massed dust revealed there. Nor is the repugnance abol- 

 ished by the reflection that, although we do not see the 

 floating particles, we are taking them into our lungs every 

 hour and minute of our lives. 



The Germ- Theory of Contagious Disease. 



There is no respite to this contact with the floating mat- 

 ter of the air ; and the wonder is, not that we should suffer 

 occasionally from its presence, but that so small a portion 

 of it, and even that but rarely diffused over large areas, 

 should appear to be deadly to man. And what is this por- 

 tion ? It was some time ago the current belief that epidemic 



