24 TEXT-BOOK OF BACTERIOLOGY. 



What processes cause this " phosphorescence " is not yet known, 

 but the researches of Lehmann and Tollhausen have made it at 

 least probable that it depends on the direct intra-cellular vital ac- 

 tion of the bacterial protoplasm, which finds expression in this pe- 

 culiar manner. The molecular changes within the cell, which in 

 other cases causes the formation of heat, of carbonic acid, etc., are 

 here accompanied by a development of light. 



The undoubted influence of changes of nourishment on the oc- 

 currence of "phosphorescence" is to be explained by the proto- 

 plasm answering such changes, sometimes with a higher, some- 

 times with a lower, degree of specific activity. 



Lastly, several of the bacteria known to us possess the power 

 of developing gas in the medium which surrounds them. With the 

 employment of solid foods this is often clearly perceptible, since the 

 bubbles of gas which are formed cannot then escape. It is the an- 

 aerobic species more especially that show a decided tendency to 

 the generation of gas. No exact investigations have as yet defined 

 the nature of the gases thus formed. 



In connection with the formation of gas may be mentioned the 

 occurrence of smells — sometimes very penetrating — with certain 

 micro-organisms. It is known to everybody that substances of 

 very offensive smell arise in the process of putrefaction. One of 

 the pigment-forming bacteria, the Micrococcus prodigiosus, when 

 grown on potato develops a decided odor of trimethylamine; others, 

 for instance, the cholera-bacillus, produce in cultivation a peculiar 

 aroma, and nearly all of the few known anaerobes have the com- 

 mon property of emitting a truly abominable stench in ordinary 

 cultivation. 



We have now completed our general observations on the bac- 

 teria. We have determined their position in the domain of nature, 

 discussed the attempts to systematize them, and made acquaint- 

 ance with their chief morphological and physiological peculiarities. 



Manifold and interesting as our attainments already are, it can- 

 not have escaped the reader that we have not yet passed beyond 

 the rudiments of an exact knowledge of the bacteria. 



Questions of extreme importance still await their solution; wide 

 fields of research, far from being sufiB.ciently examined, still lie al- 

 most untouched by our investigators. Everywhere we meet with 

 doubts and uncertainties. 



This need not be wondered at. We owe to Koch and his intro- 

 duction of the solid-transparent foods the important facts already 

 elicited and the interest now taken in the study of bacteria, al- 



