INTERMIXTURE OF EFFECTS. 529 



we tiy experiments with mercury upon a person in 

 health, in order to ascertain the general laws of its 

 action upon the human body, and then reason from 

 these laws to determine how it will act upon persons 

 affected with a particular disease, this may be a really 

 effectual method, but this is deduction. The experi- 

 mental method does not derive the law of a complex 

 case from the simpler laws which conspire to pro- 

 duce it, but makes its experiments directly upon the 

 complex case. We must make entire abstraction of 

 all knowledge of the simpler tendencies, the modi 

 operandi of mercury in detail ; our experimentation 

 must aim at obtaining a direct answer to the specific 

 question, Does or does not mercury tend to cure the 

 particular disease ? 



Let us see, therefore, how far this case admits of the 

 observance of those rules of experimentation, which it 

 is found necessary to observe in other cases. When 

 we devise an experiment to ascertain the effect of a 

 given agent, there are certain precautions which we 

 never, if we can help it, omit. In the first place, we 

 introduce the agent into the midst of a set of circum- 

 stances which we have exactly ascertained. It needs 

 hardly be remarked how far this condition is from 

 being realized in any case connected with the pheno- 

 mena of life ; how far we are from knowing what are 

 all the circumstances which pre-exist in any instance 

 in which mercury is administered to a living being. 

 This difficulty, however, though insuperable in most 

 cases, may not be so in all ; there are sometimes (though 

 1 should think never in physiology) concurrences 

 of many causes, ' in which we yet know accurately 

 what the causes are. But when we have got clear of 

 this obstacle we encounter another still more serious. 

 In other cases, when we intend to try an experiment, 



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