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 various 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 operative 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 usually 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 
togo5 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. ‘Tohis 
gas kills the roots, and, as a consequence, discoloration and 
