PEEFACE. 



THE first edition of the present work formed a part of the 

 Handbuch der Hygiene, edited by von Ziemssen and von 

 Pettenkofer ; the description, however, there given was incom- 

 plete, because it was intended to supplement it by articles by 

 other authors on " Air," " Soil," and " Common Diseases." As, 

 however, the appearance of these supplementary articles has been 

 greatly delayed, and as also, on account of the great divergence 

 of the views on the subject of micro-organisms, uniformity of 

 description was scarcely possible, I have preferred to publish the 

 second edition of " Micro-organisms " in a separate form, and to 

 add chapters on the subjects which were not treated of in the first 

 edition. 



Apart from these additions, the chapters of the previous edition 

 have been fully revised and in great part re- written. 



There are two leading ideas which have guided me in the pre- 

 paration of the present edition. In the first place, I desired to 

 give a practically useful classification of bacteria. For this pur- 

 pose I have described as fully as possible the culture characteristics 

 of the individual forms, and have provided a key for the diagnostic 

 separation of the different species of bacteria in each of the three 

 chief groups. 



I have not made any attempt to give a scientific classification 

 of the bacteria, and the characteristics described are only of use 

 practically in enabling the reader to obtain a general idea as regards 

 the species already known, and to recognise new species should 

 he meet with them, &c. In this way material will be collected, 

 which may, perhaps, at a future period be of use in leading us to 

 a truly scientific classification. 



