INTRODUCTION. 



BACTERIOLOGY is not a subject which one should attempt to 

 learn from books alone or without instructors; for aside from 

 the difficulty or impossibility of such a task, there may be 

 danger as well. Indeed the warning should perhaps be given in 

 the beginning that in view of the fact that so many bacteria cause 

 diseases which not only affect the individual who suffers, but 

 makes him a menace to those about him, no one is justified in 

 entering upon the study without proper guidance. 



Anyone who has not himself worked in a bacteriological 

 laboratory finds it difficult to form a vivid conception of what 

 bacteria are like, because among the familiar animals and 

 plants there are none with which a close comparison can be 

 made. Of the common organisms, perhaps ordinary yeasts 

 and moulds are most like the bacteria. Yeasts and moulds, as 

 everyone knows, grow on bread, cheese, meat, syrups and the 

 like. They flourish in moist and dark places, as do mush- 

 rooms, puffballs and the other fungi. All these fungi, ap- 

 pearing so different in some respects, are alike in one par- 

 ticular, which is the absence of the green color that we are apt to 

 think of as being the essential feature of vegetation. Plants 

 that are green owe their color to a substance called chlorophyll. 

 Upon the properties of this substance one of the most funda- 

 mental vital phenomena in biology depends. By means of 

 chlorophyll, under the influence of sunlight, plants are able to 

 use as food the carbon dioxide which is always present in the 

 atmosphere in small amounts. Although carbon dioxide is 

 one of the simplest and most stable of compounds, its com- 

 ponent elements are dissociated by the plant, and employed in 



