8 THE FLOATING-MATTER OF THE AIR. 



influenced probably by the great authority of Gray- 

 Lussac, fell back upon the old notion of naatter in a state 

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

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

 produced the fermentation. Bat, as a matter of fact, 

 when the plant is killed the ferment disappears. 

 Mediate or immediate, the real ' ferments ' are living 

 organisms which find in fermentable substances 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 Kircher, 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. 



