DAMAGE TO TREES BY ACID FUMES. 707 



Hess therefore considers the chemical analysis as the best 

 proof of damage done by fumes. 



The presence of sulphur dioxide in the air near the 

 furnaces is also an important factor in this question, and 

 the air and the rain and snow that fall through it should 

 therefore be analysed. 



5. Protective Measures. 



No thoroughly efficient measures have been devised against 

 this evil. Tall chimneys, sometimes 500 feet high, carry 

 the fumes into the higher strata of the atmosphere, but it 

 has been found that hydrochloric acid descends from them to 

 the ground in dense clouds, and lays waste the vegetation at 

 greater distances from the works than before. 



The best protective measure against hydrochloric acid is 

 to get it condensed, as is now done in the British Isles under 

 the Alkali Acts of 1863 and 1870, so that less than 1 per cent, 

 of the acid generated in the works escapes into the air. 



There is more difficulty in dealing with the sulphur dioxide, 

 and even the most perfect smoke-combustion cannot free the 

 air of it. Attempts have been made in Germany to convert it 

 into sulphuric acid, but this removes only one- third of the 

 injurious gas, and at Clausthal, in Germany, 1,250 tons of 

 sulphur are annually sent into the air, greatly damaging the 

 coniferous woods in the neighbourhood. 



The forester in districts where hurtful fumes exist can there- 

 fore act only by planting protective belts of strong transplants 

 of the most resisting trees in the direction of the factories, and 

 managing them entirely by the selection system. Under the 

 shelter of these belts it will be better, if possible, to grow 

 coppice or coppice-with-standards, which do not attain the 

 height of high forest. Conifers should not be grown near 

 smelting furnaces. 



6. Estimation of Damage. 



As owners of woods injured by factory fumes can claim com- 

 pensation in the courts of law, the question of estimating 

 the damage done is of great importance. 



In 1864 the Freiburg works had to pay 2,750 compensation 



