484 INDUCTION. 



stances 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 in- 

 capable of combining with the tissues, although to the 

 extent of its solubility it has a medicinal influence, 

 through an entirely different class of organic actions. 



2. The preceding instances have afforded an 

 induction of a high order of collusiveness,, illustrative 

 of the two simplest of our four methods ; although not 

 rising to the maximum of certainty 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 knowing what 

 number of real differences are involved in what is 

 nominally and apparently only one difference. It is 

 conceivable 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 however reduced in the present case to 

 almost the lowest degree possible, when we find that 

 not only one substance, but many substances, possess 

 the capacity of acting as antidotes to metallic poisons, 

 and that all these agree in the property of forming 



