246 A YEAR IN (SCIENCE 



in this way is said to be infectious. We are just 

 beginning to realize how essential it is to use all pos- 

 sible precaution to prevent the spread of diseases. Tlie 

 modern tendency in medicine is to determine how to 

 prevent diseases, rather than how to cure them. 



Decay. If any organic substance is moist, bacteria 

 will get into it from some source. They will grow 

 rapidly and in a few hours marked changes will appear 

 in the substance. The essential effect which the bac- 

 teria produce is the chemical decomposition of the 

 material upon which they are feeding. Some of the 

 simple substances formed by such decomposition are 

 consumed by the bacteria; others are not, and 

 are left behind but not in the form of the original 

 substance. This process of decomposition, brought 

 about by bacteria is called decay. As a result of it, 

 meats become putrid, eggs rot, milk sours, and fruits 

 spoil. Decay is not ahvays harmful. It is in some 

 Avays of the utmost value to man. By means of it the 

 dead bodies of animals and plants and the waste 

 products of living ones are decomposed and reduced 

 to a form in which they can be removed. The materials 

 that are broken down are thus made usable and avail- 

 able for the growth of other plants and animals. Decay 

 is essential for life. Without it, all food would finally 

 be unavailable because it would be "locked up" in the 

 bodies of plants and animals. 



To prevent the decay of food substances useful to 

 man, it is necessary to destroy the bacteria in them, 



