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117 CHAPTER V.

is a well-defined limit beyond which pure chemical methods are useless in the investigation of poisons. Chemistry can, of course, easily demonstrate to what chemical group or family a complex body belongs, and it can, therefore, afford valuable information as to its affinities. By establishing analogies in this way, chemistry often guides us to the nature of the action of a given substance, in so far as it acts like other members of the same group. But chemical investigation cannot tell us why a substance brings about a physiological effect. Chemistry can teach us that quinine and strychnia belong to the group called alkaloids, and therefore both probably possess physiological properties; but it cannot show us why one has an effect on malarious fever, and the other causes motor nerve discharges. It would, therefore, not be