452 INDUCTION. 



caustic, depriving those parts of all active vitality, and 

 causing them to be thrown off by the neighbouring living 

 structures, in the form of an eschar. The nitrate and the 

 other salts of silver ought, then, it would seem, if the theory 

 be correct, to be poisonous; yet they may be administered 

 internally with perfect impunity. From this apparent excep- 

 tion arises the strongest confirmation which the theory has yet 

 received. Nitrate of silver, in spite of its chemical pro- 

 perties, does not poison when introduced into the stomach ; 

 but in the stomach, as in all animal liquids, there is common 

 salt; and in the stomach there is also free muriatic acid. 

 These substances operate as natural antidotes, combining with 

 the nitrate, and if its quantity is not too great, immediately 

 converting it into chloride of silver ; a substance very slightly 

 soluble, and therefore incapable of combining with the tissues, 

 although to the extent of its solubility it has a medicinal 

 influence, though an entirely different class of organic 

 actions. 



The preceding instances have afforded an induction of a 

 high order of conclusiveness, illustrative of the two simplest of 

 our four methods ; though not rising to the maximum of cer- 

 tainty which the Method of Difference, in its most perfect 

 exemplification, is capable of affording. For (let us not 

 forget) the positive instance and the negative one which the 

 rigour of that method requires, ought to differ only in the 

 presence or absence of one single circumstance. Now, in the 

 preceding argument, they differ in the presence or absence not 

 of a single circumstance, but of a single substance : and as 

 every substance has innumerable properties, there is no know- 

 ing what number of real differences are involved in what is 

 nominally and apparently only one difference. It is conceiv- 

 able that the antidote, the peroxide of iron for example, may 

 counteract the poison through some other of its properties than 

 that of forming an insoluble compound with it; and if so, the 

 theory would fall to the ground, so far as it is supported by 

 that instance. This source of uncertainty, which is a serious 

 hindrance to all extensive generalizations in chemistry, is how- 

 ever reduced in the present case to almost the lowest degree 



