IN MEDICAL SCIENCE. 57 



be taken alone or in combination, in large or small 

 quantities, — are separate questions. But they are 

 elements belonging to the body, and even in mod- 

 erate excess will produce little disturbance. There 

 is no presumption against any of this class of sub- 

 stances, any more than against water or salt, provided 

 they are used in fitting combinations, proportions, and 

 forms. 



But when it comes to substances alien to the healthy 

 system, which never belong to it as normal constitu- 

 ents, the case is very different. There is a presumption 

 against putting lead or arsenic into the human body, 

 as against putting them into plants, because they do 

 not belong there, any more than pounded glass, which, 

 it is said, used to be given as a poison. The same 

 thing is true of mercury and silver. What becomes 

 of these alien substances after they get into the system 

 we cannot always tell. But in the case of silver, from 

 the accident of its changing color under the influence 

 of light, we do know what happens. It is thrown 

 out, in part at least, under the epidermis, and there 

 it remains to the patient's dying day. This is a strik- 

 ing illustration of the difficulty which the system finds 

 in dealing with non-assimilable elements, and justifies 

 in some measure the vulgar prejudice against " min- 

 eral poisons." 



I trust the youngest student on these benches will 

 not commit the childish error of confounding a pre- 

 sumption against a particular class of agents with a 



3* 



