11)07.] PUBLIC DOCUMENT— No. 33. 183 



has peculiar, characteristic odors which assist in a diagnosis. 

 Sometimes, however, especially when the tree is injured by 

 gas in late summer, at which time the flow of sap is not so 

 pronounced as in the spring, the odors of the wood are not so 

 marked. 



The writer has had many years' experience in examining 

 trees injured by illuminating gas, and has had occasion to 

 study a large number of them each year. In our diagnosis 

 of such trees Ave make use of a small hatchet, which is em- 

 ployed to cut into the trunk of a tree for the purpose of 

 examining the tissue. In most cases it is only necessary to 

 insert the hatchet into the trunk and gradually pull the tissue 

 back to see whether it is normal or abnormal. Little injury 

 is done to the tree by this practice, as a mere slit in a healthy 

 tree will heal over in a short time. In other cases it is 

 necessary to take out a chip and examine the tissues under 

 a microscope. An examination, by means of a hatchet, of 

 the larger roots which extend above the surface of the soil, 

 causes less disfigurement, and the source of leakage, if such 

 is present, may be discovered. The escape of gas into the 

 soil from a leak follows the line of least resistance. For 

 this reason, if leakage occurs in the street in front of a house 

 one can usually detect the odor of gas in the cellar, as the gas 

 will follow the exterior of the pipe leading into the cellar. 



There is considerable difference in the resistance of soils 

 to gas. In gravelly soils we have known gas to travel 2,000 

 feet without any difficulty when the ground is frozen, and 

 escape into the cellar of a house ; whereas in heavier soils gas 

 is more likely to be restricted to smaller areas. 



The poisonous properties of gas undoubtedly consist in the 

 coal tar products, which contain such compounds as sulfates, 

 cyanides, etc. Gas escaping into the soil probably condenses 

 fully as rapidly as in the pipe. The gas-drip which is taken 

 out of a pipe is the condensed portion, and this in itself is 

 very rank in odor and extremely injurious to plants, whether 

 the volatile products are taken in through the leaves or the 

 liquid through the roots. It is apparently these condensed 

 products which are taken up by the roots and which poison the 

 tree. About 1 or 2 per cent, of gas is absorbed by water, and 



