22 
IOWA ACADEMY OF SCIENCES 
This makes a list of the buildings in East Des Moines in which dishes 
were exposed. 
The number of colonies on the plates exposed in the different buildings 
in West Des Moines is as follows: 
Franklin, Room 7, 853 
Irving, Room 5 253 
Bird, Room 5, 323 
West High, Room 11, 93 
Lincoln, Room 2, 4G0 
Summit, Room 3, 235 
Forrest, Room V 355 
The incubation period in the following builuings was sixty hours in- 
stead of forty-eight: 
McKinley, Room 2 325 
Washington, Room 8, (515 
Oakland, Room 10, 425 
Garfield, Room 4 151 
Crocker, Assembly Room 108 
North High, Troutner Room 129 
Leaving Room 11 of the High school out of the calculation the average . 
number of colcnies per dish exposed in the West side buildings was S22: 
in the East side buildings G77. With these counts as a basis I find that 
just after the dust of the rooms is put into circulation by the children 
marching, the bacteria fell to the floor at the rate of 7403 on one square . 
foot of floor surface in one hour of time in the West side buildings and at 
the rate of 15570 on same area in same time in East side buildings. Con- 
siderable variation in numbers is noticeable in different buildings, the 
smallest number in any West side building being 129 the largest 615. In 
the East side buildings the corresponding numbers are 375 and 1320. « 
Of course conditions of exposure of the dishes could not be identical in 
the various buildings. The location of the building affects, in consider- 
able degree, the number of germs in the air. The fact that conditions 
under which the dishes were exposed could not be identical makes it' 
evident at once that the number of colonies in the different buildings Is ; 
not to be taken as an exact measure of the efficiency of the janitor service 
in the various buildings. Yet it does indicate '(vith a greater or less ie- 
gree of accuracy the relative condition of the rooms at the time when 
the exposures were made. The number of colonies developing in the 
dishes exposed in the Bast side buildings is approximately two times as 
great as the number of colonies developing in the dishes exposed on the 
West side. I attribute this difference to a great extent to the fact that. 
the floors of the West side buildings are oiled, while those on the east 
side are not. Bacteria are heavier than air and consequently tend to 
fall to the ground or floor. If the surface of the floor is such that the 
dust and the bacteria adhere, then movement in the room as a natural 
consequence will fail to put much of the dust into circulation. . No better 
illustration of the sticking of dust to an adhesive surface can be g'.ven 
than vTohn Tyndall’s classical experiment in 1876. If the surface of the 
