566 INDUCTION. 



is induced to do so when another oxidation is at work 

 in the midst of it. 



By the same principle Liebig is enabled to explain 

 malaria ; the pernicious influence of putrid substances ; 

 a variety of poisons ; contagious diseases ; and other 

 phenomena. Of all substances, those composing the 

 animal body are the most complex in their composi- 

 tion, and in the least stable condition of union. The 

 blood, in particular, is the most unstable compound 

 known. What, therefore, can be less surprising than 

 that gaseous or other substances, in the act of under- 

 going the chemical changes which constitute, for 

 instance, putrefaction, should, when brought into 

 contact with the tissues by respiration or otherwise, 

 and still more when introduced by inoculation into 

 the blood itself, impress upon some of the particles a 

 chemical action similar to its own ; which is propa- 

 gated in like manner to other particles, until the whole 

 system is placed in a state of chemical action more 

 or less inconsistent with the chemical conditions of 

 vitality. 



Of the three modes in which we observed in the 

 last chapter that the resolution of a special law into 

 more general ones may take place, this speculation of 

 Liebig exemplifies the second. The laws explained 

 are such as this, that yeast puts sugar into a state of 

 fermentation. Between the remote cause, the pre- 

 sence of yeast, and the consequent fermentation of 

 the sugar, there has been interpolated a proximate 

 cause, the chemical action between the particles of the 

 yeast and the elements of air and water. The special 

 law is thus resolved into two others, more general 

 than itself: the first, that yeast is decomposed by the 

 presence of air and water; the second, that matter 

 undergoing chemical action has a tendency to produce 



