228 MASS. EXPERIMENT STATION BULLETIN 170. 



gas. This protection may be secured by laying a simple device, originated 

 and used by the writer, over the gas main to convey the leaking gas to 

 certain points above the ground, thus preventing contamination of the 

 soil. By using a block system or applying it to sections 100 or 200 feet 

 long, as the case may require, and ventilating each section, a leak may be 

 readily detected and repaired before it has an opportunity to cause anj^ 

 damage. The device made of chemically treated wood and shown in 

 Fig. 97 is suitable for this purpose. It consists of pieces 2 inches square 

 in cross-section and of any desired length. This size may be adapted to 

 any size pipe and secured to it by wires at intervals of 6 to 12 feet (B). 

 Each section, which may be 200 or more feet in length, is vented by means 

 of a pipe ruiming to a pole or tree or any convenient object (D and E), or 

 may be vented directly over the pipe or near the sidewalk or curbing by 

 using an ordinary iron shield provided with vent cap, such as is used for 

 gas shut-ofl's. This takes care of all the leakage, conveying it into the 

 atmosphere at certain points. If leakage occurs it can be detected by 

 pedestrians and prevented from permeating the soil, where it would be 

 likely to kill trees on the highway. While at present it may not be feasible 

 to equip all pipe lines in this way, all new systems should be protected, 

 and those already laid as fast as possible. This protection should also be 

 extended to house services to prevent asphyxiation to human beings and 

 injury to shrubberj'- and trees on private property. 



Effects of Atmospheric Gases on Vegetation. 



The atmosphere of industrial centers is a complex mixture of various 

 substances. Besides the presence of the well-known gaseous constituents 

 found in the atmosphere, — such as oxygen, nitrogen, carbon dioxide 

 and water vapor, — hydrocarbons, solid particles and compounds of 

 carbon, nitrogen, chlorine and sulfur are present in varying quantities. 

 Argon, helium, krypton, neon, xenon and ozone are also found in the 

 atmosphere in small proportions, but so far as is known they cause no 

 detrimental effect to living organisms. Carbon dioxide, which is present 

 in the atmosphere normally ranging from .03 to.04 per cent., is not destruc- 

 tive to living organisms at this dilution. On the other hand, it furnishes 

 the most important source of food for vegetation, and plants will thrive 

 even better with a much higher concentration than that normally existing 

 in the atmosphere. Sulfur, which may be present in the air in several 

 forms, constitutes one of the most injurious agencies to plant life, and sul- 

 furous gases arising from smelters, which often contain other poisonous 

 substances, are frequently detrimental to animal life. There exists some 

 sulfur in most grades of coal, and during the process of combustion sulfur 

 dioxide is given off. This pollutes the air to a certain extent, and if 

 sufficiently abundant will injure plants. When oxygen combines with 

 sulfur dioxide, it forms suffur trioxide, which in turn forms suffuric acid 

 with water. Sulfuric acid is very corrosive, attacking and decomposing 



