DUST AND DISEASE. 313 



filled with unfiltered air, and still no trace of the beam is 

 visible. Why ? By pure accident I stumbled on this flask 

 in our apparatus-room, where it had remained quiet for 

 some time. Here are three other flasks which have also 

 been kept quiet for a couple of days ; they are all optically 

 empty. The still air of the flasks has deposited its dust, 

 germs and all, and is itself practically free from suspended 

 matter. Hence, manifestly, the result of Pasteur. 



I have had a chamber erected, the lower half of which 

 is of wood, its upper half being enclosed by four glazed 

 window-frames. The chamber tapers to a truncated cone 

 at the top. It measures in plan 3 ft. by 2 ft. 6 in., and its 

 height is 5 ft. 10 in. On the 6th of February this chamber 

 was closed, every crevice that could admit dust, or cause 

 displacement of the air, being carefully pasted over with 

 paj^er. The electric beam at first revealed the dust within 

 the chamber as it did in the air of the laboratory. The 

 chamber was examined almost daily ; a perceptible diminu- 

 tion of the floating matter being noticed as time advanced. 

 At the end of a week the chamber was optically empty, 

 exhibiting no trace of matter competent to scatter the light. 

 But where the beam entered, and where it quitted the 

 chamber, the white circles stamped upon the interior sur- 

 faces of the glass showed what had become of the dust. It 

 clung to those surfaces, and from them instead of from the 

 air, the light was scattered. If the electric beam were sent 

 through the air of the Paris caves, the cause of its impotence 

 as generator of life would, I venture to predict, be revealed. 



These experiments illustrate the application of a lumi- 

 nous beam to researches of this kind. They prove that the 

 germs which produce infusorial and fungoid life share the 

 fate of the ordinary visible dust with which they are inter- 

 mixed ; that such germs attach themselves to the sides of 

 vessels, and fall gradually to the bottom of spaces filled 

 with perfectly still air. But I will now turn to a far more 

 14 



