384 



THE AIR OF TOWNS. 



refuse, seems not unlikely ; but the subject is still in its infancy, and one 

 upon which, no doubt, fresh light will fall as bacteriological research 

 progresses. The following table gives the proportion of dust particles, 

 spores, etc., and bacteria in a cubic foot of town and country air: 1 





Average total 

 dust particles. 



Spores, 

 etc. 



Bacteria 



in 1 cubic 



foot. 





864, 000, 000 

 6, 000, 000, 000 



200 

 1,000 



2 

 20 



Town 





The numbers represent averages throughout the year, but this 

 includes considerable variations, which occur at different seasons of 

 the year. 



The shaded portion in the diagram (fig. 39) represents the number of 

 bacteria, and the dotted line the temperature during the various months 

 of the years 1879-1882. 



The number does not appear to vary proportionately with change 

 of temperature; but if we compare the rainfall with the number of 

 microbes we see at once a rapid diminution. The rain evidently car- 

 ries them down to the earth. But they are far from being destroyed. 

 The moisture seems to assist reproduction, for we find a rapid increase 

 directly after rain. If drought is long continued the number falls off 

 again. They die. Here, again (fig. 40), the shaded portion represents 

 the number of bacteria, and The line the rainfall during the year 

 1879-80. 



The number of microbes in the streets of Paris is on the average 

 about 2 L to 22 in the cubic foot, and this agrees with that found by 

 Professor Oarnelley in the streets of Dundee, viz, 20 in the cubic foot. 

 Outside of Paris the number falls off to 2 whereas, in dirty, one-roomed 

 houses Carnelley found 3,430 and Miquel in a neglected hospital ward 

 3,170 in the cubic foot. The effect of population in increasing the 

 number of microbes may be represented by the following rough map of 

 Paris (fig. 41), in which the number of microbes in a cubic meter of 

 air observed at Montsouris is marked against the arrow denoting the 

 direction of the wind. From this it will be seen that the largest number 

 occurs when the wind blows across the town and the smallest number 

 when it comes direct from the country — that is, from the south. 



The number, 21 to 22, for the streets of Paris is a rough average. In 

 dry, dusty weather, following rain, the number may rise to 150. Directly 

 after wind and rain it may fall to an average of 6 per cubic foot. 



We can not be surprised that the washings of the air by rain, accu- 

 mulating in the mud of thoroughfares, should be the gathering ground 

 for microbes. The mud of streets is more than this. It provides 

 food for their growth. It is the great source of bacterial propogation. 

 When we open our windows to let in fresh air on a dry, windy day, we 

 are welcoming these small visitors. The number of microbes in a grain 



1 Miquel. 



