x INTRODUCTION 



assist in the fixation of free nitrogen, and they build up assimilable 

 compounds. Their activity assumes innumerable phases and occupies 

 many spheres, probably more frequently proving itself beneficial 

 than injurious, for bacteria are both economic and industrious in the 

 best sense of the terms. 



Yet bacteriology has its limitations. It is well to recognise this, 

 for the new science has in some measure suffered in the past from 

 over-zealous and sanguine friends. It cannot achieve everything 

 demanded of it, nor can it furnish a causal agent for every disease to 

 which human flesh is liable. It is a science which even yet is fuller 

 of hope than of proved and established knowledge, for we are at 

 present but upon the threshold of the matter. As in the neighbouring 

 realm of chemistry, it is to be feared that bacteriology has not been 

 without its alchemy. The interpretations and conclusions which have 

 been drawn from time to time respecting bacteriological findings have 

 led to alarmist or optimist views which have not, by later 

 investigations, been fully confirmed. For the science has had devotees 

 who have fondly believed, like the Alchemists, that the twin secret of 

 "transmuting the baser metals into gold," and of indefinitely 

 prolonging human life, was at last to be known. Neither the worst 

 fears of the alarmist nor the sanguine hopes of the optimist have been 

 verified. Science does not progress at such speed or with such kindly 

 accommodation. It holds many things in its hand, but not finally life 

 or death. It has not yet brought to light either " the philosopher's 

 stone " or " the vital essence." 



What has already been said affords ample reason for a wider 

 dissemination of the elementary facts of bacteriological science. But 

 there are other reasons of a more practical nature. Municipalities 

 and other bodies are expending public moneys in water analysis, in 

 the examination of milk and the control of its supply, in the inspection 

 of cows and dairies, in the bacterial treatment of sewage, in pro- 

 tecting the oyster trade, in the ventilation of workshops and factories, 

 in disinfection, in the prevention of epidemic diseases, and in other 

 branches of public health administration. Furthermore, our increasing 

 colonial possessions with their tropical diseases, and the growth of 

 preventive medicine generally, make an increasing claim upon public 

 opinion and those engaged in raising the physical condition of the 

 people. The successful accomplishment and solution of these 

 questions depends in measure upon a correct appreciation of the 

 elements of bacteriology. 



The present is a transition period in this department of knowledge. 

 A very large body of facts has been collected, and there has been a 

 natural tendency to draw somewhat sweeping deductions which 

 subsequent knowledge has not supported. What is now required is 

 that our experience in the laboratory and outside should be patiently 



