138 FEAGMENTS OF SCIENCE. 



But as regards fermentation, the minds of chemists, 

 influenced probably by the great authority of Gray- 

 Lussac, fell back upon the old notion of matter in a 

 state of decay. It was not the living yeast-plant, but 

 the dead or dying parts of it, which, assailed by oxygen, 

 produced the fermentation. Pasteur, however, proved 

 the real ' ferments,' mediate or immediate, to be 

 organised beings which find in the reputed ferments 

 their necessary food. 



Side by side with these researches and discoveries, 

 and fortified by them and others, has run the germ 

 theory of epidemic disease. \The notion was expressed 

 by Kirch er, and favoured by Linnaeus, that epidemic 

 diseases may be due to germs which float in the at- 

 mosphere, enter the body, and produce disturbance by 

 the development within the body of parasitic life.^ 

 The strength of this theory consists in the perfect 

 parallelism of the phenomena of contagious disease 

 with those of life. As a planted acorn gives birth to 

 an oak, competent to produce a whole crop of acorns, 

 each gifted with the power of reproducing its parent 

 tree ; and as thus from a single seedling a whole forest 

 may spring ; so, it is contended, these epidemic diseases 

 literally plant their seeds, grow, and shake abroad new 

 germs, which, meeting in the human body their proper 

 food and temperature, finally take possession of whole 

 populations. There is nothing to my knowledge in 

 pure chemistry which resembles the power of propaga- 

 tion and self-multiplication possessed by the matter 

 which produces epidemic disease. If you sow wheat 

 you do not get barley ; if you sow small-pox you do 

 not get scarlet-fever, but small-pox indefinitely mul- 

 tiplied, and nothing else. The matter of. each con- 

 tagious disease reproduces itself as rigidly as if it were 

 (as Miss Nightingale puts it) dog or cat. 



