Fermentation and Kindred Phenomena. 85 



The study of the causes of putrefaction has opened up a very 

 wide field of research, and has thrown a flood of light on many 

 phenomena which were formerly hidden in mystery. For care- 

 ful enquiry has shown that the spores and seeds of minute organ- 

 isms are almost universally present, — that they occur abundantly 

 in air, water, and earth. Only some of them are concerned in 

 causing putrefaction ; others have equally well defined but to- 

 tally different functions. Thus there is a set of organisms which 

 have the power of inducing perfectly definite chemical changes 

 in certain substances, and unconsciously they have been 

 employed from time immemorial for the purpose. As an 

 example we have the organism which causes the production of 

 vinegar (acetic acid) from fermented liquids (which contain 

 alcohol). Others again cause the production of various 

 colouring matters, and some of these have not unfrequently 

 excited the awe and wonder of the superstitious. For instance, 

 there is the phenomenon of the " Bleeding Host," when bread 

 has apparently become covered with blood. But far more 

 important than any of these are the organisms which are 

 undoubtedly associated in an intimate manner with certain 

 diseases, often the very worst and most malignant to which 

 men and animals are subject. I shall endeavour presently to 

 show there are grounds for believing that in producing dis- 

 ease they are playing a chemical role, and it is by no means 

 impossible that the chemical changes induced by them in the 

 blood and secretions are the actual causes of the diseases in 

 question. Before entering upon the discussion of some of these 

 different organisms, which for our purpose we may arrange in 

 four groups, viz. : — 



(1) Putrefactive. 



(2) Chemical. 



(3) Chromogenic. 



(4) Pathogenic. 



I may be permitted to say a few words about their appearance, 



