222 Dr. a Meymott Tidy [March 2, 



I know a case, for example, where a prisoner, indicted for the 

 administration of a poison, escaped, because the scientific witness 

 declined to say that the drug administered by the prisoner was a 

 poison. 



2. The popular definition of a poison is far from being a sound, 

 much less a scientific definition. Generally speaking, it comes to this, 

 that " A poison is a drug that kills rapidly when administered in 

 a small quantity." 



The phrase " a small quantity " as regards weight, and the word 

 " rapidly " as regards time, are as indefinite as the classical piece of 

 chalk as regards size. 



I define a poison as — 



" Any substance which otherwise than by the agency of heat or 

 electricity is capable of destroying life either by chemical action on 

 the tissues of the living body, or by physiological action after absorp- 

 tion into the living system." 



(A) It will be convenient to consider first, WJiat a. poison is not. 

 It is not an agent that destroys life by physical influences, such as 



heat and electricity. 



It is not an agent that destroys life by any purely mechanical act 

 (e. g. pins are not poison, although fairly included in the phrase 

 " destructive things "). 



It is not an agent that destroys life by the mere blocking out of 

 that which is necessary to maintain life (i. c. the action of a substance 

 to be a poison must be more than mechanical). 



This latter point requires further consideration : — 



Both nitrogen and carbonic acid destroy life as certainly as they 

 extinguish a burning taper. Yet nitrogen is not a poison, whilst 

 carbonic acid is. 



Nitrogen simply destroys life by blocking out oxygen. Given 

 the presence of 20 per cent, of oxygen, the 80 per cent, of nitrogen 

 possesses no toxic effect. 



The carbonic acid, on the contrary, is specifically toxic. The 

 admixture of 20 per cent, or of 80 per cent, of oxygen does 

 not materially alter the case. Oxygen or no oxygen, CO^ is a 

 poison. 



(B) Consider next, What a poison is. 



It is an agent capable of destroying life. 



The use of the phrase deadly poison, is surplusage. If a body bo 

 a poison, it is deadly ; if it be not deadly, it is not a j^oison. 



My definition limits the mechanism whereby tbe toxic effect is 

 induced, to chemical and physiological actions. I am conscious that 

 this definition suggests classification. Certain is it, that Nature 

 hates classification as truly as she declines definitions. 



Let us trace some of these mechanisms of toxic activity. I select 

 three illustrations of poisons belonging to different classes. 



(1) Sulphuric Acid. — If a person swallows sulphuric acid, the tissues 

 with which the acid comes into contact are more or less charred :— 



