PLATE 4, 



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Agar plate exposed for 30 minutes in open air at entrance to " model dwellings " in Finsburj-, 

 London, E.G., where milk is delivered daily. Incubation, 4 days at IS' C. This plate illustrates 

 (a) pollution of air in towns ; and (h) the " raft theorj- ' of Tyndall. 



[Face page 22. 



