DUST AND DISEASE. 311 



the contrary, their author founds a defence of the doctrine 

 of spontaneous generation, and a general theory of sponta- 

 neous development. So strongly was he impressed with 

 the idea that the germs could not possibly pass through his 

 potash and sulphuric-acid tubes, that the appearance of 

 fungi, even in a small minority of cases, where the air had 

 been sent through these tubes, was to him conclusive evi- 

 dence of the spontaneous origin of such fungi. And he 

 accounts for the absence of life in many of his experi- 

 ments by resorting to an hypothesis which will not bear 

 a moment's consideration. But now that we know that 

 organic particles may pass unscathed through alkalies and 

 acids, the experiments of Dr. Bennett are precisely what 

 ought, under the circumstances, to be expected. Indeed, 

 their harmony, with the conditions now revealed, is a proof 

 of the honesty and accuracy with which they were executed. 

 On another point also the luminous beam will cast a 

 light. Pasteur opened flasks upon the Mer de Glace, and, 

 being careful not to come between the wind and his flasks, 

 found the air incompetent, in the great majority of cases, 

 to generate life. M. Pouchet repeated Pasteur's experiment 

 in the Pyrenees, adding the precaution of holding the flasks, 

 when they were opened, above his head. The luminous 

 beam at once shows us the effect of this additional precau- 

 tion. 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. Such a wind infallibly rose from 

 M. Pouchet's body as he held his flasks above his head, and 

 thus the precaution of Pasteur of not coming between the 

 wind and the flask was annulled. 



