EXAMPLES OF THE FOUR METHODS. 293 



of putrefaction, (and a fortiori incapable of the chemical actions which 

 constitute life), and the consequent is death, either of the whole organism, 

 or of some portion of it; let us compare with these cases other cases, as 

 much resembling them as possible, but in which that effect is not produced. 

 And, first, " many insoluble basic salts of arsenious acid are known not to 

 be poisonous. The substance called alkargen, discovered by Bunsen, which 

 contains a very large quantity of arsenic, and approaches very closely in 

 composition to the organic arsenious compounds 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 decomposi- 

 tion. 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 sat- 

 isfied ; for we can not be sure that these unpoisonous bodies agree with 

 the poisonous substances in every property, except the particular one of 

 entering into a diftlcultly 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 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 poi- 

 sons. For example, in case of poisoning by arsenious acid, if hydrated 

 peroxide of iron is administered, the destructive agency is instantly check- 

 ed. Now this peroxide is known to combine with the acid, and form a 

 compound, which, being insoluble, can not 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 manufactories of wliite-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 

 compounds of lead with organic matter, or of jDreventing them from being 

 formed. 



There is another class of instances, of the nature required by the Method 

 of Difference, M'hich seem at first sight to conflict with the theory. Solu- 

 ble 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 caustic, depriving those parts of 

 all active vitality, and causing them to be thrown off by the neighboring 

 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 appai-ent exception arises the strongest confirmation which the 

 theory has yet received. Nitrate of silver, in spite of its chemical proper- 

 ties, does not poison when introduced into the stomach ; but in the stom- 

 ach, as in all animal liquids, there is common salt; and in the stomach 

 there is also free muriatic acid. These substances operate as natural anti- 

 dotes, combining with the nitrate, and if its quantity is not too great, im- 

 mediately converting it into chloride of silver, a substance very slightly 



