98 FERMENTS AND MICRO-PARASITES. 



in animal organs, do not of necessity apply to man, and 

 it was therefore necessary, when possible, to bring 

 forward experimental facts which would undoubtedly show 

 that micro-organisms are the direct and only cause of 

 certain diseases, and are not merely accidental com- 

 panions of other noxious materials. 

 Definite proof Experiments were formerly performed with this aim 

 tne following manner : inoculations were carried out 



micro- w ith infective substances, an effort being made at the 



organisms by - . , 



isolating them same time to free the organisms from other materials 



infective ex- adhering to them, which might possibly cause the 



periments. disease. In some cases an attempt was made to isolate 



the organisms by the addition of distilled water, in 



which the organisms would sink to the bottom, or by 



filtration ; but in these attempts it was always question- 



able whether the noxious materials by possibility present 



in solution were really removed, and on the other hand 



whether the organisms were not injured by the washing 



isolation by and by the great osmosis. Filtration in the living 



body, obtained by studying the relation of the foetus to 



the infected maternal organism, did not lead to a definite 



result, as it was only in certain instances (anthrax) that 



the foetus remained free from infection, while in other 



cases it also became attacked by the disease. 



By dilution of Experiments were then made by dilution of the 

 matSiaf. tlve infective material, on the assumption undoubtedly 

 correct that only a contagium dependent on a living 

 organism capable of multiplication could be diluted to 

 an extreme degree without losing its activity. Such a 

 dilution was in reality obtained when one animal was 

 successfully inoculated from an infected one, a third from 

 the second, and so on through a whole series of animals ; 

 nevertheless there was always the possible objection in 

 this case that the cells of the body might take part in 

 the regeneration of the poison. 



On the other hand all doubt as to the pathogenic 

 properties of certain micro-organisms is removed in the 

 face of the proof obtained during the last few years, that 

 the infective material can be diluted outside the body 

 to an extraordinary extent, without thereby losing its 

 activity. Thus Koch was able to dilute infective blood 



