FERMENTATION. 277 



has been most strictly demonstrated. But we now pass 

 the bounds of surgery proper, and enter the domain of 

 epidemic disease, including those fevers so sagaciously 

 referred to by Boyle. The most striking analogy be- 

 tween a contagium and a ferment is to be found in the 

 power of indefinite self-multiplication possessed and 

 exercised by both. You know the exquisitely truthful 

 figures regarding leaven employed in the New Testa- 

 ment. A particle hid in three measures of meal lea- 

 vens it all. A little leaven leaveneth the whole lump. 

 In a similar manner, a particle of contagium spreads 

 through the human body and may be so multiplied as 

 to strike down whole populations. Consider the effect 

 produced upon the system by a microscopic quantity of 

 the virus of smallpox. That virus is, to all intents and 

 purposes, a seed. It is sown as yeast is sown, it grows 

 and multiplies as yeast grows and multiplies, and it 

 always reproduces itself. To Pasteur we are indebted 

 for a series of masterly researches, wherein he exposes 

 the looseness and general baselessness of prevalent 

 notions regarding the transmutation of one ferment 

 into another. He guards himself against saying it is 

 impossible. The true investigator is sparing in the use 

 of this word, though the use of it is unsparingly ascribed 

 to him ; but, as a matter of fact, Pasteur has never 

 been able to effect the alleged transmutation, while he 

 has been always able to point out the open doorways 

 through which the affirm ers of such transmutations 

 had allowed error to march in upon them. 1 



The great source of error here has been already 



1 Those who wish for an illustration of the care necessary in 

 these researches, and of the carelessness with which they have in 

 some cases been conducted, will do well to consult the Rev. W. H. 

 Dallinger's excellent ' Notes on Heterogenesis ' in the Octobel 

 number of the Popular Science Review. 



