THE MICROBIOLOGY OF THE ATMOSPHERE 



but some flasks remained sterile. In cellars of the Observatoire, where the 

 temperature was constant and the air still and dust-free, many more 

 flasks remained sterile. 



On 5 November i860, Pasteur deposited at the office of the Academy- 

 no fewer than seventy-three quarter-litre flasks, some of which he had 

 opened to the air in batches of twenty at various heights ranging from the 

 foothills of the Jura to high up on Mont Blanc, as follow s : 



Number of flasks 

 Altitude Locality where air sampled Contaminated Sterile 



Country air, far from dwelling 

 houses, on the first plateau of 

 the Jura 8 12 



850 metres Jura mountains 5 15 



2,000 metres Montanvert, near Mer de 



Glace on Mt. Blanc i 19 



The cause of this supposed 'spontaneous generation' was not only 

 discontinuous but, moreover, its concentration decreased with height. 



F. A. Pouchet had admitted that among dust particles of vegetable 

 origin there were some spores of cryptogams, but he held that these 

 were too few to account for the phenomena of putrefaction. 



Pasteur decided that he would abandon Pouchet's method, which 

 relied on examining spontaneous deposits of dust on the surface of objects, 

 in favour of a new method of studying the particles by collecting from 

 actual suspension in the air. Pouchet had drawn invalid conclusions from 

 surface deposits because, according to Pasteur, the light air-movements 

 which constantly play over surface deposits would pick up and remove 

 the extremely minute and light spores of microbes more readily than they 

 would any coarser particles. (It now appears, however, that the small 

 numbers of the lighter bodies in surface deposits is due to the extreme 

 slowness with which they are deposited, rather than to their preferential 

 removal after deposition.) 



Pasteur's apparatus for extracting the suspended dust in the air, for 

 microscopic examination, was quite simple (Fig. i). A tube of | cm. 

 diameter was extruded into the open air through a hole drilled in a 

 window frame several metres above the ground. The rear part of the tube 

 was packed with a plug of gun-cotton to catch particles. Air was drawn 

 through the apparatus by means of a filter pump, and the volume of air 

 was measured by displacement of w^ater. Tests were made on air draw^n 

 from beside the Rue d'Ulm, and from the garden of the Ecole Normale 

 in Paris. During aspiration, solid particles were trapped on the fibres of the 

 gun-cotton plug. After use, the gun-cotton was dissolved in an alcohol- 

 ether mixture, the particles were allowed to settle, the liquid was decanted, 

 and the deposit was mounted for microscopical examination. 



4 



