WHAT WE BREATHE 35 



ridicule his conclusions by declaring that, were 

 this the case, the air must of necessity be so 

 heavily laden with living forms that we should be 

 surrounded by a thick fog " dense comme du 

 fer." We do not now, forty years later, require to 

 recite the exquisitely simple experiments which, 

 whilst sufficiently establishing his theories, served 

 to effectually suppress those of his opponents. 



Since Pasteur's pioneering work was carried out, 

 a vast number of investigations have been made 

 in all parts of the world by scientists of almost 

 every nationality on the subject of the distribution 

 of bacteria in air, and not only on their distribu- 

 tion, but on their functions or the place they 

 occupy in the economy of nature. With our 

 increased knowledge concerning their distribution 

 has come our ability to differentiate between 

 individuals, and to adequately assess the value and 

 importance of their work from various points of 

 view. 



In the bacterial treatment of sewage we have 

 not only one of the latest, but perhaps also one of 

 the most successful examples of that system of 

 division of labour, or specialisation of energy, 

 which forms such a characteristic feature of work 

 of all kinds at the present time Other familiar 

 instances of the applications of individual and 

 special bacterial labourers to the solution of in- 



