244 PROTECTION OF WOODLANDS. 



CHAPTER V. 



PROTECTION AGAINST DAMAGE BY SMOKE AND OTHER 

 ATMOSPHERIC IMPURITIES. 



122. Occurrence of Damage ly Smoke and Gases. 



For a very long time it has not failed to be noted that the 

 smoke from factories and smelting-works, and under certain 

 circumstances even from railway-engines when frequently passing 

 to and fro, proves prejudicial to the well-being and normal 

 development of woodlands subjected to their influence, producing 

 changes from the natural colour in the foliage, a sickening and 

 dying-off of many kinds of plants, and. particularly those of 

 sylvicultural importance. In the great majority of cases the 

 cause of the injury lies in the sulphurous acid contained in 

 the smoke, for only in less frequent cases can it be proved to be 

 due to the action of hydrochloric acid in a gaseous form as 

 evolved in the manufacture of soda, and to arsenious or nitrous 

 acids. 



The sulphurous acid is imbibed in its gaseous form through 

 both the upper and the under sides of leaves and needles, and 

 then by oxidation is quickly transformed into sulphuric acid. 

 Props of fluid water passing over the leaves, as during rainfall, 

 hastens the action of the acid in a considerable degree, without, 

 however, being concerned in the origin of the damage, that is, in 

 the absorption of the sulphurous gas. Under the action of the 

 sulphuric acid formed, the spines or needles become first of all 

 yellow or red at their points, occasionally with a sharply drawn 

 distinguishing line between the still sound green portion and the 

 part affected, which latter gradually extends until the whole 

 spine changes colour and becomes dead. On the foliage of broad- 

 leaved species of trees the action of the sulphuric acid, ilmi 

 is being formed, makes the leaves appear as if they had been 



