378 A SHORT HISTORY OF SCIENCE 



The nineteenth century began as a period of agnosticism in 

 pathology. The older theories were discredited, but beyond a 

 general belief in the material basis of the agencies of disease 

 almost nothing was known. Scurvy, indeed, had been shown 

 to be due to lack of certain kinds of food, and smallpox had been 

 proved to be preventable both by inoculation and vaccination. 

 Boyle, in the seventeenth century, had ventured the guess that 

 diseases might be "fermentations," but as fermentations were 

 not yet understood the suggestion had little value. Light finally 

 came from two sources, viz., from parasitology and from zymology, 

 the science of fermentations. It had long been recognized that 

 the mistletoe was a parasite causing serious disease in trees, 

 and that tapeworms might cause disease in animals which they 

 infested. It was not, however, until the microscope came into 

 use that the itch, long known as a contagious disease, was found 

 to be due to a parasitic insect, while flies, fleas, bedbugs and lice 

 were still thought to be annoying rather than dangerous. The 

 discovery in 1837 that "honeycomb" of the scalp (Favus), an 

 infectious disease in which yellow crusts appear on the head, is 

 due to a vegetable parasite related to the moulds was a surprise, 

 as was the demonstration in 1839 that an infectious disease 

 (Muscardine) of silkworms is likewise due to a mould. 



The microscope also served to reveal what have been called 

 " the footprints of disease " within the cells and tissues, making 

 possible the work in cellular pathology by Virchow. 



THE GERM THEORY OF FERMENTATION, PUTREFACTION AND 

 DISEASE. PASTEUR. At about this time the achromatic com- 

 pound microscope was coming into use, and by its aid the alco- 

 holic fermentation, hitherto regarded as a purely chemical phe- 

 nomenon, was found to be intimately associated with, if not 

 actually caused by, a living, growing microorganism, yeast, ob- 

 served and figured by Leeuwenhoek in 1680, but in the early 

 nineteenth century regarded rather as potent organic matter in 

 some peculiar catalytic state or condition (Liebig) than as a 

 living thing. Between the rediscovery of yeast in 1837 and 

 Pasteur's epoch-making studies upon it in 1859, fermentation was 



