302 DISEASES OF TREES 



spruces that are still living, though fully exposed in the 

 neighbourhood of blast furnaces, it will be found that it is only 

 on the youngest shoots that the leaves are still green. The 

 farther one moves from the seat of the mischief, so do the 

 annual crops of leaves that still maintain their position on the 

 spruce-shoots increase. It is thus evident that the duration of 

 the leaves depends in large measure on the intensity of the 

 action of the smoke. Amongst dicotyledonous trees the beech 

 is the most sensitive, after which come the oak and the 

 sycamore, while the elm, ash, and mountain ash, and, amongst 

 conifers, the black pine are some of the most resistant. In 

 towns where it is only in winter that large quantities of coal 

 are used as fuel, the conifers alone suffer. In summer the air 

 is almost free from sulphurous acid, and it is only on the 

 approach of cold weather that the deleterious influences begin 

 to make themselves manifest. At this time the deciduous trees 

 have shed their leaves, so that it is only the conifers that are 

 affected. The sulphurous and sulphuric acids that collect in 

 large quantities in snow that has covered foliage for some time 

 prove injurious to the trees. 



The ease with which sulphurous acid is oxidized to hydrated 

 sulphuric acid not only explains how this plant-poison is 

 constantly being removed from the atmosphere, but also 

 indicates how we may remove sulphurous acid from the smoke 

 of blast furnaces and factories generally. To some extent this 

 has already been put into practice. By leading the sulphur 

 gases through moistened hydrated lime 90 per cent, is rendered 

 innocuous. Another plan is to conduct the gas through a long 

 pipe in which a stream of water flows in the opposite direction. 

 By this process a conversion into hydrated sulphuric acid is 

 effected. 



According to recent observations the chlorine and soda fumes 

 that are produced in certain factories also prove injurious to 

 vegetation. 



THE EFFECTS OF LIGHTNING 



Up to the present the way in which lightning affects the 

 health of trees remains unexplained. 



When lightning strikes a wood, its effects may be confined to 



