DUST AND DISEASE. 35 



revealed by the luminous beam. He prepared twenty- 

 one flasks, each containing a decoction of yeast, filtered 

 and clear. He boiled the decoction so as to destroy 

 whatever germs it might contain, and while the space 

 above the liquid was filled with pure steam, he sealed 

 his flasks with a blow-pipe. He opened ten of them in" 

 the deep, damp caves of the Paris Observatory, and 

 eleven of them in the courtyard of the establishment. 



Of the former, one only showed signs of life su^gjg-" 



quently. In nine out of the ten flasks no organisms of 

 any kind were developed. In all the others organisms 

 speedily appeared. 



Now here is an experiment conducted in Paris, on 

 which we can throw light in London. Causing our 

 luminous beam to pass through a large flask filled 

 with the air of this room, and charged with its germs 

 and its dust, the beam is seen crossing the flask from 

 side to side. But here is another similar flask, which 

 cuts a clear gap out of the beam. It is filled with un- 

 filtered 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. Acting upon this obvious suggestion, I set 

 aside three other flasks, filled, in the first instance, with 

 mote-laden air. They are now optically empty. Our 

 former experiments proved that the life-producing 

 particles attach themselves to the fibres of cotton-wool. 

 In the present experiment the motes have been brought 

 by gentle air-currents, established by slight differences 

 of temperature within our closed vessels, into contact 

 with the interior surface, to which they adhere. The 

 air of these flasks has deposited its dust, germs and all, 

 and is practically free from suspended matter. 



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

 of wood, its upper half being enclosed by four glazed 



D 2 



