THE MONTHLY BULLETIN. 



245 



EXTENT OF INJURY. 



While it is not at all difficult to detect cases of severe injui-y where 

 vegetation is constantly exposed to the action of the gas, there is some- 

 times eon.si(l(>i-alile difficulty experienced in detecting injury where such 

 takes place intermittently. Thus close to a smelter in the direction of 

 the prevailing winds there are always areas where injury is very marked, 

 and may be seen at all times, and other places farther away where 

 injury takes place periodicall\- and if not observed just at the right time 

 may escai)e detection entirely. To determine the' limits of damage is 

 consequently a very "hard problem. 



There are a number of factors, such as moisture, winds, canyons and 

 air currents which are active in determining the extent and severity of 

 the injui-y done. In sections where damage is being done it occurs in 



Fig. 49. — Leaf of large-leaved maple 

 injured by sulphur dioxide gas liberated in 

 tlie smelting of ores. (Original.) 



its most exaggerated form during damp weather; winds may cause a 

 rapid dissipation of the fumes and a lessening of the chances of injury. 

 Canyons and air currents or drafts created by them may carry the fumes 

 for great distances, causing severe injury wherever they may happen 

 to settle. It is not at all an uncommon thing to be able to detect the 

 odor of sulphur dioxide gas in the air at a distance of twenty miles or 

 more from its source of liberation, and it is probable that there is always 

 sufficient gas present to cause more or less injury when i.t may be 

 detected by the sense of smell. Such injury may not always manifest 

 Itself in such a definite way as has been described, but rather in the 

 bringing about of a general weakening of plants because of exposure 

 to the action of small quantities of the gas from time to time, or an 

 obscure injury not easily detected. 



