244 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



an arbitrary weight as the standard ; thus for i gram of soil the 

 number of milligrams of various substances added would be : — 



Molecular weight, Molecular weighty Molecular weight 



10 50 

 Molecular weight Molecular weight 



100 200 



or, as it is usually and more briefly expressed, 



M, M/10, M/50, M/100, M/200, &c. 



Counts are now made of the numbers of the organisms present in 

 each lot of soil and the limit is found beyond which the organism will 

 not survive. Thus the organism may be apparently unaffected by 

 the M/200 dose, not much reduced by the M/100 ; it may be seriously 

 diminished by the M/50 dose and completely suppressed by the M/10 

 dose. The toxic dose therefore lies between M/10 and M/50, and would 

 be expressed as M/10 to M/50. 



A derivative of the substance is then made by the chemist by 

 introducing a new group ; the effect of the new substance is then 

 tested. 



Supposing the toxic dose is now M/10 to M, the new substance is 

 obviously less effective than the old. The chemist therefore tries 

 in some other direction. Supposing the new group increases the 

 toxicity so that the toxic dose becomes M/50 to M/100, this represents 

 a distinct improvement. The chemist therefore goes further in the 

 same direction and introduces another group of the same kind. The 

 toxicity may be still further increased. A third group is then intro- 

 duced ; there may be a still further increase, or, on the other hand, 

 no increase at all in toxicity. 



In this way numbers of groups can be examined and their effects 

 ascertained. As an example, the effect on the wireworm of certain 

 groups, ascertained by Messrs. Tattersfield and Roberts, may be 

 quoted : — 



Amounts Required to Kill Wireworms. 



(Gram Molecular Weights.) 



Basal Substance. 



Added Group. 



One Group. 



Two Groups. 





Methyl 



54 



30 





Chlorine 



26 



8 





Bromine 



14 



6 





Benzene 



Iodine 





100 



Amide 



35 



Non-toxic 





Nitro 



3 



Non-toxic 





Hydroxyl 



i-4 







Chlormethylene 



o-5 





One of Mrs. Matthews' tables for eelworm, fungi, and protozoa 

 is as follows : — 



