39 



vent leakage and injury to trees, and some of our more progressive 

 gas manufacturers spare no expense or skill in constructing and 

 maintaining their lines. In laying the larger pipes, which are more 

 difficult to keep calked securely, they are providing their patrons 

 with better facilities ; nevertheless, they run a greater risk from 

 leakage. 



Numerous connections are found in gas mains from which can be 

 detected only slight leakage, perhaps only a few cubic feel a day ; 

 whereas there are others from which the leakage is extensive. The 

 presence of small leaks, if not attended to, will injure trees 

 in the course of time, since the soil becomes charsed with gas 



Fig. 19. Large elms killed by escaping illuminating gas, one and one-half years after 

 leakage occurred. From "Park and Cemetery." 



to a greater or less extent in a few years. 



In the eastern states the three principal kinds of gas used are 

 water, coal and oil gas, and so far as the effects of these various 

 gases on trees are concerned there is apparently little or no differ- 

 ence, since they all contain similar elements which are poisonous to 

 trees. 



Two degrees of injury may readily be distinguished as resulting 

 from gas poisoning ; first, incipient cases, and second, pronounced 

 cases. In the first series we have those already alluded to as result- 

 ing from small leaks, the soil in such cases not becoming saturated 



