EXAMPLES OF THE FOUR METHODS. 451 



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 antecedent which 

 we had already good ground for considering as the proximate 

 cause. 



But the rigorous conditions of the Method of Difference 

 are not yet satisfied ; for we cannot be sure that these un- 

 poisonous bodies agree with the poisonous substances in every 

 property, except the particular one, of entering into a difficultly 

 decomposable compound with the animal tissues. To render 

 the method strictly applicable, we need an instance, not of a 

 different substance, but of one of the very same substances, in 

 circumstances 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 suboxide, neither 

 of which enters into combination with animal matter. The 

 disease called painter s colic, so common in manufactories 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 sul 

 phuric acid has the property of decomposing all compounds 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 con 

 flict with the theory. Soluble salts of silver, such for instance 

 as the nitrate, have the same stiffening antiseptic effect on 

 decomposing animal substances as corrosive sublimate and 

 the most deadly metallic poisons; and when applied to 

 the external parts of the body, the nitrate is a powerful 



292 



