THE NATURE OF BACTERIA. 37 



upon and within the soil, or in bodies of water. The simpler 

 molecules produced by the decomposition of the more complex 

 substances are indefinite in number, and are understood by the 

 chemist in a few cases only. The chemistry of the decom- 

 position of organic substances is still in its infancy and as yet 

 only the general nature of the changes is understood. The 

 decomposition of these compounds in general brings the ele- 

 ments back to simpler conditions, and nearer to the form in 

 which they can serve as food for ordinary plants. 



Both synthetical and analytical processes are carried on, to 

 a certain extent, by all bacteria. If they grow and multiply 

 they must be manufacturing proteid and protoplasm out of the 

 food products, for each new bacterium is made of protoplasm. 

 This is, of course, a synthetic process and is characteristic of 

 all bacteria. On the other hand, all bacteria likewise produce 

 a certain amount of decomposition of the materials which serve 

 them as food, giving rise to simpler products as excretions. 

 But while all bacteria thus perform both types of chemical 

 change, some classes give rise, as the total result of their life, 

 to a marked decomposition of complex bodies, while others 

 produce a marked synthesis as the total result of their chemical 

 activity. The latter are few compared to the former. 



We must notice that in still another respect the chemical 

 changes produced by bacteria are two-fold.' In some cases 

 the new products which arise are of the nature of excretions. 

 By this is meant that certain substances are taken into the 

 bacteria and then subjected to a series of changes within their 

 bodies. These changes are classed together under the name 

 of metabolism. As a result of the metabolic changes there 

 arise new chemical products which are eventually eliminated 

 from the body of the bacteria as excretions. Some of the 

 products arising in a mass of organic material undergoing de- 

 composition by bacteria are thus of the nature of excretions 

 (e. g., ptomaines]. But in other cases the new chemical bodies 



