THE ORIGIN AND PROPAGATION OF DISEASE. 237 



others showed that the germs of the yeast-plant maybe disseminated 

 by the atmosphere, and that the same precautions which exckide the in- 

 troduction of germs from without into a fermentable liquid also exclude 

 the process of fermentation itself; so that we can now accept with confi- 

 dence the double fact — 1. That the growth and reproduction of the yeast- 

 fungus will take place only in a fermentable liquid ; and, 2. That such a 

 liquid will ferment only Avheu the yeast-fungus is present and in a state 

 of active development. 



The revolution in opinion on this point was so complete that, in regard 

 to the alcoholic fermentation at least, its essential results were finally 

 accepted by Liebig himself. In his last treatise on fermentation, x^ub- 

 lished in 1871, he says: * "There no longer remains any doubt as to the 

 nature of the ferment of beer and wine. It is a cryptogamic vegetation, 

 more or less fully developed * * * We may conclude that the 

 albuminous matters of the yeast take part in its action upon the sugar, 

 and it is evident that these albuminous matters acquire their property of 

 exciting fermentation by becoming an actual constituent of the yeast 

 itself." 



Consequently, the fermentation of a saccharine liquid is the result of 

 vegetative activity. We add to the liquid a few cells or spores of the 

 yeast-fungus. These grow and multiply, and the turbidity of the liquid 

 is due to their increase and dissemination. They decompose its sugar, 

 appropriate some of its elements, and leave as a result alcohol and car- 

 bonic acid. When all the sources of their nourishment are exhausted, 

 fermentation stops, and the liquid becomes clear, the j^east-cells subsid- 

 ing to the bottom. But the ferment has in the mean time been repro- 

 duced, like so much grain which has been sown, raised, and harvested ; 

 and a little of the deposit left at the bottom of the vessel, if introduced 

 into another saccharine liquid, vfill in turn reproduce the i^rocess of fer- 

 mentation. 



It is impossible not to perceive a certain analogy between the general 

 phenomena of fermentation and those of contagious and infectious 

 diseases. The period of incubation which intervenes between the ex- 

 posure to a contagion and the appearance of the malady — the regular 

 course of the symi)toms — their natural termination within a definite time, 

 and the evident reproduction of the contagious element — all these facts 

 were so many points of resemblance, which could not escape the atten- 

 tion of medical observers. The analogy, indeed, has long been recog- 

 nized in our nomenclature ; and the term zymotic diseases cannot mean 

 anything else than diseases depending upon some cause which acts after 

 the manner of a ferment. But this name was adopted only as a matter 

 of convenience, and was understood altogether in a symbolical sense. 

 Of late we have begun to suspect that after all it may be simjDly the 

 expression of a literal fact. 



A similar order of discoveries has recently been made with regard to 



* "Auuales de Chimie et de Physique/' 1871, tome sxiii, pp. 9, 10. 



