98 THE AMEEICAN MONTHLY [May, 



extending about exactly one inch. On the table they are mainly shown 

 in company with the red blood globules so that their relative size may 

 be appreciated. The blood globules are individually about -g-^Vcr "^^ ^^ 

 inch in diameter. 



Bacteria, like other living beings, perform all the functions of organic 

 life ; they feed, excrete, multiply their numbers, and produce elaborate 

 chemical products, in many cases of a highly poisonous nature. Those 

 living in free air, and feeding upon carbo-hydrates, produce, as a rule, 

 little or no elaborately poisonous products ; those living in confined 

 spaces apart from air, and feeding upon flesh-forming or nitrogenous 

 compounds, tend to produce poisons more or less deadly. These are 

 the bacteria that are fitted to live in the animal body, to poison it and to 

 create diseases of a contagious nature. Then, again, dift'erent bacteria 

 require for their active life different amounts of air, heat, light, and 

 electricity, and if these are widely different from those found in the 

 animal body, they cannot live in that body and become a contagion. 

 Others require special chemical conditions of their food, and if they can- 

 not tind these in the animal body, they cannot fix themselves upon it as 

 parasites. As an example, most of animal juices are alkaline or neu- 

 tral, and can only maintain bacteria that normally live in alkaline or 

 neutral solutions. If a germ requiring an acid liquid is introduced, it 

 can only survive in the acid contents of the stomach or large intestines, 

 or in the acid secretions of the skin or open sores. For the same reason 

 the alkaline-feeding bacteria, if taken with food, are, to a large extent, 

 destroyed in passing through the acid stomach, which thus acts as a 

 protective sentinel, keeping guard over the intestinal canal. It is only 

 during disease' of the stomach, or in the absence of its acid contents, 

 that these bacteria pass in scatheless. When, however, the bacteria 

 have formed spores, these, like dried seeds, may safely pass through the 

 acid stomach and germinate in the intestine. 



Asfain, bacteria are the common scavengers of the universe. We 

 have long known that plants and animals reciprocate with each other 

 in producing each the food required by the other. Plants take up sim- 

 ple soluble and gaseous materials and build them up into complex com- 

 pounds fit for the food of animals. Animals, on their part, break down 

 these complex compounds, and furnish them again to the plants in the 

 simple forms available for their food. In the case of carbon dioxide, 

 ammonia, and some other forms this is true, but after this there is still 

 a large body of animal products that are not soluble in water and not 

 available for plant food. Those it is the function of the bacteria to 

 transform and prepare. They are the cooks of the vegetable creation. 

 Every fermenting manure heap, every rotting vegetable and animal, is 

 a great kitchen in which this preparation of vegetable food is going on. 

 But for the constant beneficent work of the bacteria the world would 

 soon be choked up with theundecomposed remainsof plantsand animals, 

 and vegetable and animal life must alike perish. They are at once the 

 scavengers, caterers, and cooks of nature, then, and as no living beings 

 are so widely distributed, so no living beings are more beneficent in 

 their work. When covered or preserved foods fail to keep it is because 

 we have failed to kill all the bacteria ; when we make bread, beer, wine, 

 sauerkraut, and other common products, we harness these infinitesimal 

 beings and employ them for our uses. 



