94 PASTEUR: THE HISTORY OF A MIND 



doned towers of old cathedrals, and at the bottom of the 

 hypogea in ancient Egypt. Futile pains! It is not the 

 dust which falls and is deposited that interests us! You 

 will find therein only the heaviest parts of what the wind 

 carries, mineral particles, grains of starch or of pollen, 

 the spores of cryptogams or even bits of down, of cotton, 

 of wool from the living sheep or from our garments. It 

 is not these particles which we must study, but rather 

 those which we see dancing without repose in a ray of 

 sunlight, and which the air contains in the state of a 

 permanent suspension." 



"Furthermore, your study of the dust of cathedrals 

 gives you no indication of quantity. What is the volume 







FIG. 9. Dust of the air caught by aspiration in the meshes of gun- 

 cotton. 



of air which has deposited the little mass which you have 

 studied, and subjected to microscopic examination? 

 You do not know, and consequently your experiments 

 may well open the question, but they do nothing to solve 

 it." 



"Nevertheless how easy the thing is! Let us take 

 the cotton filter of Schroeder and Dusch, and replace 

 it only by gun-cotton, and when by it we have arrested 

 in its passage the dust in a determined volume of air, 

 let us throw the gun-cotton into a mixture of alcohol and 

 ether in which it is soluble. All the weft of the filter 

 is dissolved. The particles of dust which have been 

 caught in the meshes are set at liberty and fall to the 



