PROTECTION AGAINST DAMAGE BY SMOKE, ETC. 245 



tattooed with lighter and darker reddish-brown spots, which 

 discoloration, in the case of extensive damage, gradually spreads 

 until the green colouring totally disappears, and the leaf dies 

 altogether. When the injury to the foliage is attributable to 

 hydrochloric acid, the leaves are always characterised by a 

 discoloration of their edges. 



Coniferous species of trees are most sensitive to the injurious 

 effects of smoke and other atmospheric impurities, and, from self- 

 evident causes, are so in the order represented by the duration of 

 their spines or needles, viz., Silver Fir, Spruce, Scots Pine, Larch. 

 By reason of their annual change of foliage, the broad-leaved 

 species are altogether less sensitive ; the Oak and the Plane 

 being those which suffer least, then Maple and Sycamore, Ash, 

 Elm, Poplar, and Mountain Ash, whilst Alder, Lime, and Horn- 

 beam are more sensitive, and Beech apparently the most sensi- 

 tive of all. The plants best endowed with a capacity for with- 

 standing the effects of impurities appear to be those of 

 agricultural growth and also vegetables. 



The damage inflicted is always greatest where narrow valleys 

 almost force the currents of air to set in particular directions, or 

 where the injurious acids and impurities occur largely in the 

 atmosphere. But the local conditions of climate also exert 

 a certain influence in the matter, as the damage is enhanced in 

 places with damp, foggy atmosphere always hanging about them. 



The crops injured first of all show a diminution in their 

 increment, and an increased mortality among individual trees, so 

 that the canopy first of all gets interrupted, and, in the case 

 of large quantities of impurities working on sensitive species 

 of trees, gradually more and more broken, till finally the whole 

 crop dies off, and vacant spaces are formed, which are uncommonly 

 hard to re-stock. Young pole-forests of 15 to 30 years of age are 

 liable to suffer most from atmospheric impurities. 



123. Preventive Measures. 



Various plans have from time to time been adopted, but 

 hitherto without much success, for minimising the damage thus 

 caused, and all the more so as in Germany the owners of the 

 factories are held liable to pay compensation for the injury done. 

 Endeavours to transform the sulphurous acid contained in the 



