382 THE AIR OF TOWNS. 



It would be easy to multiply examples to which this great discovery 

 has given rise. Tuberculosis, diphtheria, wool-sorter's disease, leprosy, 

 cholera, typhus, aud tetanus have been traced to the existence of micro- 

 scopic living matter (figs. 33, 34). 



I have taken you over this little bit of history in order to indicate 

 the importance of knowing the number and character of the almost 

 invisible living germs of the air, and this must be my apology for intro- 

 ducing a subject which may seem to lie a little outside the special topic 

 of town air. 



If we examine dust under a powerful microscope, we find that it con- 

 sists of a variety of things, which I enumerated in my last lecture. 

 Now, the greater part of this dust, although heavier than air and set- 

 tling rapidly where the air is still, is so very fine as to be almost invis- 

 ible except when illuminated by a bright beam of light. Can we gain 

 any idea of the weight of these little particles ? In my second lecture, 

 I told you that the weight of dust in 100 cubic feet of air in town is over 

 1 milligram. In my last, that in the parish churchyard 3,638,000 dust 

 particles were contained in 1 cubic inch. From this it may be calcu- 

 lated that about 40 million million dust particles weigh 1 grain and 

 would occupy a space of 240 cubic yards, or a space measuring rather 

 over 6 yards each way. 



What proportion of the dust consists of spores, pollen, and fungi, and 

 what proportion do the bacteria form? The amount of living matter 

 in the air has been carefully investigated for a long period of years by 

 M. Miquel, of the Observatory of Montsouris, situated on the outskirts 

 of Paris. This careful experimenter has directed bis attention mainly 

 to determining the number of vegetable spores, fungi, and microbes in 

 the air in various places and at various seasons of the year. He has 

 determined the amount of vegetable matter and microbes in the streets, 

 bedrooms, and living rooms of Paris and in the environs. He has drawn 

 samples of air from the sewers of Paris, and from the top of the Pan- 

 theon, high above the town. He has examined the street dust, the dust 

 of rooms, of the soil in the country, and in graveyards — in short, the 

 dust of all possible places where disease germs might lie. Anyone who 

 has leisure to take up the book "Les Organismes Vivants" can not fail 

 to be interested in the results of so much laborious work and of so many 

 carefully recorded facts. It would take too long and carry me beyond 

 my subject, if I gave even a brief outline of these results; I must limit 

 myself to town air and the minute organisms which inhabit it. The 

 vegetable spores and fungi we may pass over briefly. This slide (fig. 

 35) represents the most common forms met with at the Montsouris 

 Observatory. 



