THE AIR OF TOWNS. 



383 



And the next table the average number throughout the year for the 

 years 1878 to 1882 in 1 cubic foot. 1 



January 200 



February 200 



March 155 



April 212 



May 346 



June 992 



July 786 



August 677 



Sep tember 450 



October 406 



November 252 



December 200 



The average for the country is 200 and for the town 1,000. This refers 

 to Paris, where there are plenty of beautiful parks and where trees line 

 the larger streets. Where vegetation is nearly obliterated, as in the city 

 of Leeds, the number will probably fall much below that of the country. 



We now come to the much more minute inhabitants of the dust — the 

 microbes or bacteria. Here are some of the commoner forms as seen 

 under a powerful microscope (fig. 36). 



There are globular and elongated forms, twisted filaments, spherical 

 dots, and short, straight rods. Yeast cells, too, are often met with. 

 They rapidly reproduce; the parent cell in the case of bacteria divid- 

 ing into two or more new cells, and these again undergoing subdivision. 



It may interest you to know how these almost invisible germs can 

 be counted. Although the germ itself is only visible under high mag- 

 nification, if the germ falls upon nutrient material it will soon produce 

 a family circle readily visible as a spot of mold. One of the methods, 

 which has been introduced by a German bacteriologist named Hess, is 

 represented in the following diagram (fig. 37). 



It consists of a glass tube coated with a nutrient jelly. The tube 

 is first rendered sterile by heat, and then a measured volume of air is 

 slowly aspirated through it by the aid of two bottles containing water, 

 which can be alternately lowered and raised. The tube is then placed 

 under the best conditions for the growth of the germs and excluded 

 from the dust. Where a germ has fallen a spot of mold will soon 

 appear, and such spots mark the residence of the original single germ. 



The following slide (fig. 38) represents the appearance produced in 

 the tube in three experiments made in a schoolroom: No. 1 experiment 

 was made before the school assembled, the second in the middle of the 

 day, and the last when the school closed. 



One is struck by the great variety of these minute beings, and the dif- 

 ficulty of distinguishing them is increased by the fact that they appear 

 to vary in shape with the nutrient material upon which they grow. If 

 they are fed on beef tea they may take a different shape to that pro- 

 duced by a diet of agar jelly. There seems very little doubt that the 

 number of species is very large, and very little is known, moreover, of 

 their functions. It is certain that at least a few produce disease. It 

 is equally certain that a large number, when inoculated into animals, 

 are harmless. That these harmless ones serve a useful purpose in car- 

 rying on putrefactive change, acting as scavengers for the world's 



Miquel. 



