44 



The particular eftucls ul individual iiienibers of the groups are 

 limited by their physical properties such as volatility, etc., which 

 may be regarded as indirect consequences of their chemical 

 constitution. 



The aromatic hydrocariions and halides are on the whole more 

 loxic tlurn the aliphatic hydrocarbons and halides. The groups 

 that influence toxicity most when introduced singly into the ben- 

 zene ring are in order of importance the methylamino (most effec- 

 tive) , dimethylamino, hydroxy, nitro, amino, iodine, bromine, 

 chlorine, methyl groups (least effective). But this order is modi- 

 fied in presence of another group ; thus when there is a CHg already 

 present in the ring the order becomes chlorine (side chain) , amino, 

 hydroxy, chlorine (ring), methyl. Chlorine and hydroxy groups 

 together give rise to highly poisonous substances considerably 

 more effective than where present separately. The association of 

 chlorine and nitrogroups in chlorpicrin give rise to one of the most 

 toxic substances tested. Methyl groups substituted in the amino 

 group of aniline increase toxicity more than if substituted in the 

 ring. 



Conipoujids with irritating vapours have usually nigh toxic 

 values, e.f;., Allyl isothiocyanate, chlorpicrin, benzyl chloride. The 

 toxic values of these substances are not closely correlated with their 

 vapour pressures or rates of evaporation. 



There is 'A fairly close relationship between toxicities and the 

 vapour pressure, rates of evaporation and volatilities of compounds 

 of the same chemical type. In a series of similar compounds de- 

 creases in vapour pressure and volatility are associated with an 

 increased toxicity. A possible explanation is that condensation or 

 absorption takes place along the tracheal system when insects are 

 submitted to the action ol these vapours. On exposure once more 

 to the open air these vapours diffuse out into the atmosphere, the 

 rate at which they do so being a measure of the rapidity w-ith 

 which the insect recovers. 



A limit is put upon toxicity by the decrease in vapour pressure, 

 when it sinks too knv to allow a toxic concentration in the vapour 

 phase. Chemically inert compounds boiling above 170° C. are 

 generally uncertain in their poisonous effects on wireworms after 

 an exposure of 1,000 minutes at 15" C. Nearly all organic com- 

 pounds boiling above 215*^ C. are uncertain in their action, while 

 those boiling above 245" C. are non-toxic. These limits depend on 

 the resistance of the insect, the length of exposure and the tem- 

 perature at which the experiment is carried out. 



XXII. X. N. Sen Gupta. " DephenuUsatioji in Soil/' 

 Journal of Agricultural Science, 1921. Vol. XI. 



It is found that phenol and the cresols disappear when added 

 to soil. Three actions seem to be involved : — 



1. — An instantaneous disappearance which appears to be non- 

 biological, but its exact nature has not yet been elucidated ; 

 apparently it varies directly with the clay content of the soil. 



2. — A slow decomposition which continues till all the phenol is 

 exhausted. This is apparently largely brought about by micro- 

 organisms capable of utilising phenol as a source of energy. 



