LETHAL GASES 193 



500 hours of exposure. One realizes the great resistance of animals as com- 

 pared with the most sensitive plants when it is stated that leaf tissue in 

 buckwheat is killed by seven hours' exposure to 0.46 ppm, though animals 

 are not injured by 500 hours' exposure to 33 ppm. At high concentrations 

 a longer total exposure to gas was necessary to cause death when the SO2 

 treatments were given in intermittent doses. 



Fig. 73 shows the length of exposure to various concentrations of SO2 

 necessary to kill 50 per cent of mice and guinea pigs. Above 200 ppm the 

 guinea pigs are more resistant than the mice, and below that concentration 

 the mice are much more resistant than the guinea pigs. Fifty per cent of 

 the mice were still alive after nearly 900 hours in 150 ppm. The suscepti- 

 bility of both grasshoppers and cockroaches approximated that of the mice. 



Symptoms in vertebrates at higher concentrations of SO 2 were lethargy, 

 nasal catarrh, lachrymation, coughing, conjunctivitis, difficult breathing, 

 distension of abdomen, weakness, and paralysis of hindquarters. In the 

 highest concentration insects cleaned their mouth parts and antennae, 

 showed lack of coordination of muscular movements, and paralysis of 

 posterior legs. The authors describe the internal pathologic changes in 

 mammals resulting from high dosages: ^^' p-^'"^ "Pathologic changes in 

 vertebrates include general visceral congestion of slight to moderate degree, 

 slight to moderate edema of the lungs ^^^th hemorrhages at higher concen- 

 trations, acute dilation of the right heart at higher concentrations, gross 

 distention of the stomach with multiple ulcers and hemorrhages at the 

 higher concentrations, distention of the gall bladder except at lowest 

 concentrations." 



Effect of Hydrogen Sulphide on Plants 

 In connection with spray injury from Ume sulphur, McCallan and 

 associates " ran continuous-flow fumigation experiments with hydrogen 

 sulphide on 29 different species of plants. The fumigations were carried on 

 in shaded glass cases outside during the summer. The duration of the 

 fumigation was five hours in each case and was carried on at midday. The 

 mean temperature was 74° to 81° F (23° to 27° C) and the relative humidity 

 from 82 to 100 per cent in the several experiments. 



The symptoms of injury were scorching of young shoots and leaves and 

 basal and marginal scorching of next older leaves with the older and mature 

 leaves unaffected. This differs strikingly from the injury by SO2 which kills 

 the parenchyma of the middle-aged leaves in lower fumigations while the 

 young and mature leaves are more resistant and the stems uninjured. 



The authors describe further results of their experiments: '^' p-^^^^" 

 "The different species varied \\idely in their response: carnation, purslane, 

 Boston fern, apple, cherry, peach, strawberry, and coleus showed no 

 appreciable injury at concentrations below 400 ppm; pepper, rose, nastur- 

 tium, castor bean, gladiolus, sunflower, buckwheat, and cornflower, slight 

 to moderate injury at concentrations from 40 to 400 ppm; and soybean, 



