HISTORICAL INTRODUCTION 



Fig. I. — Pasteur's gun-cotton filter for airborne microbes. 



a = gun-cotton plug, i cm. long, held in position by: 



b = spiral platinum wire. 

 FF = window frame drilled to allow passage of: 

 T = tube to exterior for sampling outdoor air. 



R (m.k.l.) = aspirator. 



Pasteur, as usual, had little interest in the specific identity' of his or- 

 ganisms; he was no taxonomist. The particles exactly resembled the 

 'germs' of lower organisms. They differed in volume and structure so 

 much among themselves that they clearly belonged to very many species 

 or even groups, including bacteria, moulds and yeasts. Their numbers 

 contradicted the general conclusion that the smallest bubble of air admitted 

 to a heat-sterilized medium is sufficient to give rise to all the species of 

 infusoria and cryptogams normal to an infusion. This view was sho\\Ti to be 

 highly exaggerated, and Pasteur indicated clearly that it is sometimes 

 possible to bring a considerable volume of ordinary air into contact with an 

 infusion before living organisms develop in the latter. 



Pasteur had demonstrated visually the existence of an air-spora, he 

 had pointed out that it should be measured while in suspension and not 

 after deposition on surfaces, and he had made the first rough visual 

 measurements of its concentration in the atmosphere of the City of 

 Paris: a few metres above the ground in the Rue d'Ulm, after a succession 

 of fine days in summer, several thousands of micro-organisms were 



