304 THE FLOATING-MATTEB OF THE AIR. 



flasks to take in air without contracting contamination. 

 A quarter of a century ago Pasteur proved the cause of 

 ' so-called spontaneous generation ' to be discontinuous. 

 I have already referred to his observation that 12 out 

 of 20 flasks opened on the plains escaped infection, 

 while 1 9 out of 20 flasks opened on the Mer de Glace 

 escaped. Our own experiment at the Bel Alp is a 

 more emphatic instance of the same kind, 90 per cent. 

 of the flasks opened in the hayloft being smitten, while 

 not one of those opened on the free mountain ledge was 

 attacked. 



The power of the air as regards putrefactive infec- 

 tion is incessantly changing through natural causes, and 

 we are able to alter it at will. Of a number of flasks 

 opened in 1876 in the laboratory of the Eoyal Institu- 

 tion, 42 per cent, were smitten, while 58 per cent, es- 

 caped. In 1877 the proportion in the same laboratory 

 was 68 per cent, smitten, to 32 intact. The greater 

 mortality, so to speak, of the infusions in 1877 was due 

 to the presence of hay which diffused its germinal dust 

 in the laboratory air, causing it to approximate as re- 

 gards infective virulence to the air of the Alpine loft. 

 I would ask my friend to bring his scientific penetration 

 to bear upon all the foregoing facts. They do not 

 prove spontaneous generation to be ' impossible.' My 

 assertions, however, relate not to 'possibilities,' but to 

 proofs, and the experiments just described do most 

 distinctly prove the evidence on which the heterogenist 

 relies to be written on waste paper. 



My colleague will not, I am persuaded, dispute these 

 results ; but he may be disposed to urge that other able 

 and honourable men working at the same subject have 

 arrived at conclusions difi"erent from mine. Most freely 

 granted ; but let me here recur to the remarks already 

 made in speaking of the experiments of Spallanzani, 



