34 THE FLOATING-MATTEE OP THE AIE. 



cution of his experiments, and in the reasoning based 

 upon them, is perfectly evident to those who, through 

 the practice of severe experimental inquiry, have ren- 

 dered themselves competent to judge of good experi- 

 mental work. He found germs in the mercury used to 

 isolate his air. He was never sure that they did not 

 cling to the instruments he employed, or to his own 

 person. Thus when he opened his hermetically-sealed 

 flasks upon the Mer de Glace, he had his eye upon the 

 file used to detach the drawn-out necks of his bottles ; 

 and he was careful to stand to leeward when each flask 

 was opened. Using these precautions, he found the 

 glacier air incompetent, in nineteen cases out of twenty, 

 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 efifect. 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 

 imdemeath 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 



