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-question : Have these organisms any existence in tlie living 

 body, and, if so, have tliey any special role either in health or 

 disease ? This is a question easier to ask than to answer. "We 

 are liable to errors, both of observation and of interpretation ; 

 but, so far as our best observers, aided by the most perfect 

 microscopical appliances at command, have been able to learn, 

 there is a general agreement — (1) That Bacteria do not exist 

 in the fluids or tissues of an animal in health ; (2) that in 

 some diseased conditions, and particularly in diseases of an 

 infectious nature, Bacteria can, as a rule, be found in abun- 

 dance ; (3) that in some of these diseases the Bacteria can be 

 distinguished one from another by special modes of observa- 

 tion ; (4) that they can be further distinguished by culture in 

 sterilized fluids ; (5) that the Bacteria cultivated in such fluids 

 have the property of communicating a disease to healthy animals 

 exactly resembling that affecting the animal from which the 

 infecting material was obtained; and (6) that it is possible by 

 modes of culture, and other means, to modify the virulence of 

 some of the most dangerous Bacteria to such a degree that 

 when fluid containing them is inoculated into a healthy animal 

 it produces but a slight temporary effect, but sufiicient to pro- 

 tect the animal for an indefinite period from further infection. 

 These are the revelations of modern science. Let it be under- 

 stood that I do not claim that all these points have been 

 ascertained with respect to all infectious diseases. There is 

 no need to overstate the case. I shall hope to show you that 

 in some of these diseases the facts are established beyond a 

 doubt, and that reasoning by analogy we are justified in the 

 inference that what is true of some is probably true of the 

 whole class. 



Time will not permit me to take up the several points I have 

 enumerated in regular series. I prefer, therefore, to discuss 

 them generally ; but before doing so I ask you to allow me to 

 introduce a lesson in microscopical technology, for the want 

 of which many well-meaning observers have fallen into errors. 

 Many of the objects we are dealing with are so extremely 

 minute that when magnified under glasses, which give distinct 

 images of objects magnified in the proportion of an inch 

 square to a square of 50 yards, do not appear larger than pins' 

 heads, and even when so magnified would probably be invisible 

 if not stained by a process perfected by Koch in his researches 

 on infectious diseases. Objects so minute require special 

 methods of examination, and I think Koch and workers of like 

 rank are justified in refusing to notice the objections of less 

 practised observers, who have neglected to follow the special 

 means which tkey believe to be essential for successful demon- 

 stration. Koch insists on the use of oil immersion lenses, of 



