26S 



INDUCTION. 



found in the body, has not the slightest 

 injurious action upon the organism," 

 Now when these substances are brought 

 into contact with the tissues in any 

 way, they do not combine with them ; 

 they do not arrest their progress to 

 decomposition. As far, therefore, as 

 these instances go, it appears that 

 when the effect is absent, it is by 

 reason of the absence of that ante- 

 cedent which we had already good 

 ground for considering as the proxi- 

 mate cause. 



But the rigorous conditions of the 

 Method of Difference are not yet 

 satisfied ; for we cannot be sure that 

 these unpoisonous bodies agree with 

 the poisonous substances in every pro- 

 perty, except the particular one of 

 entering into a difficultly decompos- 

 able compound with the animal tis- 

 sues. To render the method strictly 

 applicable, we need an instance, not 

 of a different substance, but of one of 

 the very same substances, in circum- 

 stances which would prevent it from 

 forming, with the tissues, the sort of 

 compound in question ; and then, if 

 death does not follow, our case is made 

 out. Now such instances are afforded 

 by the antidotes to these poisons. 

 For example, in case of poisoning by 

 arsenious acid, if hydrated peroxide 

 of iron is administered, the destructive 

 agency is instantly checked. Now this 

 peroxide is known to combine with 

 the acid, and form a compound, which, 

 being insoluble, cannot act at all on 

 animal tissues. So, again, sugar is 

 a well-known antidote to poisoning 

 by salts of copper ; and sugar reduces 

 those salts either into metallic copper, 

 or into the red sub-oxide, neither of 

 which enters into combination with 

 animal matter. The disease called 

 painter's colic, so common in manu- 

 factories of white lead, is unknown 

 where the workmen are accustomed 

 to take, as a preservative, sulphuric 

 acid lemonade (a solution of sugar 

 rendered acid by sulphuric acid). 

 Now diluted sulphuric acid has the 

 property of decomposing all com- 

 pounds of lead with organic matter, 



or of preventing them from being 

 formed. 



There is another class of instances, 

 of the nature required by the Method 

 of Difference, which seem at first 

 sight to conflict with the theory. 

 Soluble salts of silver, such, for in- 

 stance, as the nitrate, have the same 

 stiffening antiseptic effect on decom- 

 posing animal substances as corro- 

 sive sublimate and the most deadly 

 metallic poisons ; and when applied 

 to the external parts of the body, the 

 nitrate is a powerful caustic, depriv- 

 ing 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 

 exception arises the strongest confir- 

 mation which the theory has yet 

 received. Nitrate of silver, in spite 

 of its chemical properties, 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 com- 

 bining 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 influ- 

 ence, through an entirely different class 

 of organic actions. 



The preceding instances have afford- 

 ed 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 Differ- 

 ence, in its most perfect exemplifi- 

 cation, is capable of affording. For 

 (let us not forget) the positive in- 

 stance and the negative one which the 

 rigour of that method requires, ought 

 to differ only in the presence or ab>- 



