LETHAL GASES 



199 



of yellow or reddish brown due to the presence ^ of tannins in the leaves. 

 Darkness also protected leaves from injury by NH3 but to a lesser degree 

 than it did against SO2. The petioles of the younger leaves ^* were first 

 killed by HCN. In the higher dosages CI2 and SO2 showed marked acidula- 

 tion of the leaves, CI2 being more effective than SO2, whereas H2S caused 

 only slight acidulation. Ammonia made the leaves more alkaline. The pH 

 of the soil was lowered by CI2 and SO2 and raised by ammonia. Stems were 

 much less sensitive on the whole than leaves, wdth httle if any significant 

 difference between the several gases. 



The toxicity of the five gases to houseflies and mammals shows a great 

 difference in effectiveness \\dth no overlapping of results. They rank as 

 follows: HCN > H2S > CI2 > SO2 > NH3. 



Table 21 shows (A) order of toxicity of gases to classes of organisms, and 

 (B) order of sensitivity of classes of organisms to the five different gases 

 based on the highest concentration used, 1000 ppm. In general, CI2 and 

 SO2 are most toxic to plants and HCN and H2S to animals. 



Table 22. Time Till 50 Per Cent Mortality of Gas-Treated Animals 



(In Minutes) 



Up to now we have discussed the effect of only one concentration, 

 1000 ppm, mth only the time of exposure varying. If the total dosage 

 (time X concentration) were a constant for producing a given physiological 

 effect, this would be adequate at least for one kind of organism. Calculating 

 from the figures in Table 22 will show that the product law does not hold. 

 There is another reason why various concentrations as well as various times 



