110 MASSACHUSETTS HORTICULTURAL SOCIETY. 



and bacteria, both needing a certain amount of moisture and food 

 to propagate. Dry bread will not mould ; moulds must have 

 moisture to increase, but they require very little food. Probably 

 there is food enough for them in any room, however clean. In 

 August we finds moulds springing up everywhere, showing that 

 the germs are everywhere. 



Mineral particles are found in great excess in the air of cities. 

 One investigator tried by the process of fog-making to estimate 

 the number in a cubic inch. In a rainy day he found 521,000 ; Id 

 fair weather 2,119,000 ; in the air of rooms 30,318,000, and near 

 the ceiling 88,346,000 in a cubic inch. This dust is not usually 

 dangerous. 



It is only quite recently that processes have been devised for 

 the determination of the number of living organisms or of their 

 spores in the air. Those interested in the subject will find a full 

 discussion in Mr. G. R. Tucker's admirable paper on the number 

 and distribution of micro-organisms in the air of the Boston City 

 Hospital, printed in the Twentieth Annual Report of the Massachu- 

 setts State Board of Health. 



All investigators agree that while the particles of dust are so 

 abundant in out-door air the number of living organisms is very 

 small, ten litres (about two gallons) of air containing as a role 

 from ten to fifteen, and this holds good for Dundee, Scotland, and 

 London, England, as well as for Boston. 



The number increases near the ground ; for instance. Dr. Percy 

 Frankland found that the air taken at the height of the top of the 

 dome of St. Paul's, contained only ten organisms in ten litres, 

 while air from the base gave thirty-five, and that from the surface 

 of the ground gave forty-seven organisms for the same amount. 



But when houses and public buildings are examined, the results 

 are quite difl!"erent. Professor Carnelly found at Dundee, Scot- 

 land, that the air of the more roomy and clean houses contained 

 from 160 to 180 micro-organisms in ten litres, while that of the 

 smaller and dirtier houses had from 400 to 930. 



Mechanically ventilated schools gave much better results, the 

 cleanest having «nly thirty organisms and the worst only 300, 

 while in some of the schools for poorer children there were 910, in 

 schools of average cleanliness 1,250, and in dirty schools 1,980» 



It was supposed that the number of organisms would depend 

 wholly on the cleanliness of the rooms, but it was found that 



