Minimal Lethal Dose of various Toxic Substances. 321 



the difference in their size or relative area of alimentary surface as suggested 

 by Moore. 



In proceeding to discuss our own examination of the subject of the 

 present communication, we begin by endeavouring to show how the dose of 

 a given poison which kills animals of a particular species in a given time is 

 related to the weight of the individual. 



For this purpose certain experiments with diphtheria toxin, the full 

 details of which will be found in a subsequent paper ("An Analysis of the 

 Problem of the Minimal Lethal Dose, etc."), are made use of, and some of the 

 results obtained are given below. 



In the case of diphtheria toxin, if the death time (3£ days) which is 

 usually taken in the standardisation of this substance be made use of, it is 

 found that for guinea-pigs which die in about 80 hours the lethal dose 

 expressed as a percentage of the weight works out as follows : — 



For animals of between — 

 200 and 250 grin, weight, about 6.0 cu. mm. of Toxin B per 100 grm. 

 310 „ 370 „ „ 5-2 



415 „ 530 „ < 5-0 



Again, taking a death time of some 40 hours, it is found that in the 

 lightest group of guinea-pigs the minimal lethal dose per 100 grm. of weight 

 is about 6 - 5 cu. mm. of the toxin, in the group of medium weights it is about 

 6 - 2 cu. mm., while in the heaviest group it is about 6 cu. mm. 



Hence it follows that, for individuals of differing weights, the minimal 

 lethal dose cannot be rightly expressed as a percentage of the body weight. 

 This fact is, of course, well known to those familiar with the routine 

 estimation of toxicity. 



In Tables I and II are given two groups of animals, the one group 

 consisting of light individuals and the other of heavy ones ; where the 

 dosage expressed in per cent, of body weight was approximately the same. 



The average weight of the animals in Table I is 234 grm., the average dose 

 per 100 grm. is 6 - 3 cu. mm., the dose estimated in relation to the surface area, 

 and calculated from the expression D = d\ W°' 72 > where d is the actual quantity 

 of the drug introduced, is 291 cu. mm., and the average time to death is 

 46 hours. 



The average weight of the animals in Table II is 425 grm., the average dose 

 per 100 grm. is again 6 - 3 cu. mm., but the dose calculated in relation to the 

 surface area has increased to 34"3 cu. mm. and the average time to death is 

 seen to be reduced to 37 hours. It is, therefore, evident that when the 

 dose per 100 grm. of weight is made the same in light and heavy groups of 



