DAMAGE TO TREES BY ACID FUMES. 697 



been arrived at in the case of coal-smoke. In order to 

 ascertain the fact, various species of woody plants have been 

 subjected to frequent and prolonged exposure to artificially 

 produced fumes of each of the separate components of the 

 smoke. Sulphur dioxide in the soil has no prejudicial effects 

 on plants, as has been proved by watering them with diluted 

 sulphur dioxide solution, for the gas speedily becomes converted 

 into sulphuric acid, and forms harmless compounds with 

 alkalies in the soil. 



The action of the sulphur dioxide when the air is moist, or 

 the leaves moistened with dew or rain, is rapid and decisive ; 

 it is probably absorbed by plants in the form of sulphuric 

 acid, being taken up in variable quantities by the leaves or 

 needles of different species of trees. It then proceeds from 

 the leaves into the twigs. The leaves or needles gradually 

 turn brown, owing to the decomposition of the chlorophyll 

 and tissues of the leaf. 



Sulphur dioxide finds its way into the atmosphere by the 

 roasting of minerals containing sulphur, and from coal-fires, 

 coal containing about 2 per cent, of sulphur, chiefly in the 

 form of iron-pyrites. What minute quantities of this gas 

 suffice to kill plants was proved in 1864 by experiments with 

 spruce plants which were exposed to air containing only one- 

 millionth part of sulphur dioxide. After 335 puffs of the air, 

 the points of the needles began to turn brown, and eventually 

 turned completely brown. 



It has also been supposed that the soot in smoke might 

 injure forest trees by blocking up their stomata, but this 

 mode of injury is not admitted by Stockhardt. 



As injurious compounds of the smoke of other works may be 

 reckoned : vapours of mercury,* hydrochloric acid gas, oxides 

 of nitrogen, and chlorine, also steam containing soda particles 

 from cellulose-factories. The influence of hydrochloric acid 

 from alkali-works is shown whenever the air contains O'l 

 per cent, by a considerable increase in the chlorine in the 

 leaves. Leaves get brown or red edges, and eventually dry-up 

 and fall. Chlorine acts similarly, but more energetically. 

 Hydrochloric acid is very destructive to vegetation, sometimes 



* Wagner's " Jhrbch.," 1874, p. 277. 



