ON DUST AND DISEASE. 165 



to generate life ; while similar flasks, opened amid the 

 vegetation of the lowlands, were soon crowded with 

 living things. M. Pouchet repeated Pasteur's experi- 

 ments in the Pyrenees, adopting the precaution of 

 holding his flasks above his head, and obtaining a 

 different result. Now great care would be needed to 

 render this procedure a real precaution. The luminous 

 beam at once shows us its possible effect. Let smoking 

 brown paper be placed at the open mouth of a glass 

 shade, so that the smoke shall ascend and fill the shade. 

 A beam sent through the shade forms a bright track 

 through the smoke. When the closed fist is placed 

 underneath the shade, a vertical wind of surprising 

 violence, considering the small elevation of temperature, 

 rises from the hand, displacing by comparatively dark 

 air the illuminated smoke. Unless special care were 

 taken such a wind would rise from M. Pouchet's body 

 as he held his flasks above his head, and thus the pre- 

 caution of Pasteur, of not coming between the wind 

 and the flask, would be annulled. 



Let me now direct attention to another result of 

 Pasteur, the cause and significance of which are at once 

 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 subse- 

 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 



