226 Dr. a Meymott Tidy [March 2, 



noted that their toxicity was inversely to their atomic weight, 

 fluorine (19) being the most poisonous, and iodine (127) the least 

 poisonous of the group, chlorine (35 • 5) and bromine (80) occupying 

 their proper intermediate positions. But here again the group 

 theory was inevitable. What was true of monad elements was not 

 true of the elements of higher atomicity, the toxicity of selenium (79) 

 being far greater than that of sulphur (32). 



With these facts before us there arises this question, Was a 

 relationship to be expected between physiological action and atomic 

 weight ? One poison acts on muscles — a second on nerves and nerve- 

 centres — a third on the blood : — Is it likely, even supposing a relation- 

 ship to exist between a certain group of elements and a given organ 

 or a given structure, that the relationship would be the same in the 

 case of all organs and all structures? These researches (the oiitline 

 of which I have briefly indicated) suggest this much to future observers, 

 viz. : First of all group your poisons according to their methods of 

 operation, and then see how far the degree of toxicity of the terms of 

 any one group show relationship to the atomic weights of such group. 



But the difficulties of comparison thicken, when we consider the 

 physiological action of certain allotropic modifications of the 

 elements. 



Thus compare yelloiu phosphorus, a body readily inflammable, 

 soluble in bisulphide of carbon, firing by contact with iodine, with 

 red phosphorus, a body at variance with the yellow variety in the 

 three respects named. Nor is this all. For yellow phosphorus is an 

 active poison, two grains being a certainly toxic dose, whilst red 

 phosphorus is an absolutely inert body. 



Take a second illustration. In its ordinary form oxygen plays 

 the part of a life sustainer. But oxygen is only a life sustainer in its 

 common form and at ordinary pressure. 



On this latter point I have no time to dwell, save to mention that 

 if an animal be exposed to oxygen at three pressures, the resulting 

 symptoms are not unlike those induced by strychnine. 



There is, however, an allotropic form of oxygen called ozone. 

 The physiological action of ozone was the subject of a communication 

 to the Royal Society of Edinburgh by Dcwar and McKendrick (1873). 

 Their experiments were made on both cold and warm-blooded animals, 

 including amongst the latter themselves and their assistants. 



The results are remarkable, more particularly when we remember 

 that the air with which they operated, at most only contained 10 ])ev 

 cent, of ozone. 



Placing a large frog in a jar of air, and then ozonizing the air, 

 the frog in about half a minute closed its eyes, the respirations fell 

 from 96 to 8 per minute, and the body temperature was lowered 4° or 

 5° C. The animal recovered in about 8 minutes, when pure air was 

 admitted into the receiver. Death resulted if the animal was exposed 

 for any lengthened period to the action of the ozonized air. 



As regards the action of ozone on warm-blooded animals, Dewar 



