IOWA ACADEMY OF SCIENCES. 
77 
in the rural districts, where no attention was given to the 
examination and purification of the water, as in the city, 
where municipal hygiene has been given a fair considera- 
tion. All the larger cities and most of the smaller ones 
have provided themselves with municipal laboratories in 
whichfchemical and bacteriological examinations are con- 
stantly being made. It stands to reason that in the city 
with onejsource of water supply, it would be much easier 
to examine and purify the water than to examine as many 
sources'of water as there are families scattered over a large 
territory. It is the duty of the city to take advantage of 
its hygienic opportunities, because it is not true that the 
death rate is necessarily higher in the city than in the 
country. 
A comparatively few deaths in one place attract a great 
deal more attention than many deaths scattered over a 
large territory. The six hundred people killed at the burn- 
ing of the Iroquois made a profound impression on our 
country and the result has been the enactment and en- 
forcement of laws to prevent its recurrence. While the 
one hundred thousand people that die annually from the 
effects of tubercle bacteria scarcely receive a passing notice. 
Perhaps no other idea has so influenced and moulded our 
modern civilization as the scientific knowledge of the 
infinitesimal little plants known as bacteria. In the phys- 
ical world we have to do with three forces. First, forces 
between masses, or gravitation; second, forces between 
molecules, or cohesion and adhesion; third, forces between 
atoms, or chemical affinity. Upon first thought it might 
be supposed that the greatest of these forces is gravity, but 
a little reflection will soon convince one that it is the weak- 
est. Suspend a pencil between the fingers. The force of 
gravity is unconsciously overcome. To overcome the force 
of cohesion, it will take considerable effort, but it may be 
fairly well done by breaking the pencil in two. To over- 
come the chemical force that exists between the atoms of 
the pencil no known mechanical force is great enough. A 
thousand tons might be thrown against it and crush it to 
