70 Diagnosing of Diseases 



Chlorine gas, developed by the burning of coal in smelters 

 and in certain soda factories, as well as soluble metallic 

 oxides from \'arious manufactures, have similar effects. 



Only the systematic suppression of the smoke nuisance, 

 which several cities, notably St. Louis, have begun, and the 

 enforcement of factory laws (such as are o])erative in most 

 European communities) to prevent the escape into the air 

 of poisonous substances, will remove this cause of disease 

 for which there is no remedy. 



The escape of illuminating gas produces similar effects. 

 To injure foliage a large amount of this gas is required, 

 such as the continuous vitiation of the air, which often takes 

 place in the neighborhood of gas works or of natural gas 

 wells. The effect is usuall}- the dying of portions of leaves, 

 the tips and margins especially turning brown and dry; 

 sometimes the entire leaf becomes dry and withered, hang- 

 ing, however, tightly to the branch. 



More frequently still does the escape of gas into the soil 

 from leaky pipes do harm. Indeed, when we read that in 

 1905 the gas companies of Massachusetts experienced a 

 loss of ten per cent, of the gas actually produced, we must 

 conclude that gas leaks are a much more frequent cause of 

 damage to trees in streets and parks than is usually sup- 

 posed. 



It is probably the sulphuretted hydrogen, which, taken 

 up by the roots, acts as a poison. The result of gas poison- 

 ing may be only a partial or local dying of parts, namely, 

 of those directly affected by the leak; but it may also cause 

 .death of the whole tree. 



The presence of the gas is detected by the odor when the 

 ground is turned, and by a blue coloration of the roots or 

 of the sap-wood, ascertained by boring or chipping. This 

 gas kills the roots, and, as a consequence, discoloration and 



