THEIR NATUEB AND EFFECTS. 117 



CHAPTER V. 



THE NATUEE OF SNAKE-POISONS. 



There 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 informa- 

 tion 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 in- 

 vestigation 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 physio- 

 logical 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 reason- 



