696 PROTECTION AGAINST DISEASES. 



Fruit-trees exposed to acid fumes cease bearing fruit before 

 the foliage is seriously injured, and St. Helen's was formerly 

 famed for its fruit; but since 1867, owing to the chemical 

 works in its neighbourhood, no fruit has been produced there. 

 Crops of wheat exposed to acid fumes may to all appearance 

 be ripe and full when scarcely a grain is to be found in the 

 ears. Eoot-crops, such as potatoes and turnips, suffer less, 

 and on the whole trees suffer much more than grass or 

 agricultural crops. 



In the Tavistock woods, large areas of oak coppice were 

 seriously injured by sulphurous fumes from arsenic mining 

 works. Owing to the diminished working of these mines, it 

 is now possible to replant the injured area with oak and 

 larch. 



In 1861, extensive damage was found to have been done to 

 coniferous woods by the fumes from the works at Freiburg, in 

 Saxony, in some of which sulphuric acid is made from iron 

 pyrites. Stockhardt* and Schroder,! at the Tharandt 

 laboratory, investigated the chemical components of the 

 smoke which cause the damage, and Hamburger | has done 

 the same more recently. Subsequent notices^ have appeared 

 about damage in the Oberharz owing to acid fumes, and the 

 area of forest damaged by three iron- works, in 1881, was 

 about 11,250 acres. In the Altenau forest-range this damage 

 has become noticeable since American ores have been smelted, 

 which contain more sulphur than the native ores. 



Even by smoke from charcoal kilns Scots pine needles 

 have been reddened at a distance of 50 yards, but this is due 

 to the heat of the smoke. 



2. Injurious Components of Smoke. 



It has been proved by observations made at Tharandt, that, 

 of the components of the fumes from the Saxon Works, lead, 

 arsenic, and sulphur compounds, soot, etc., only sulphur 

 dioxide is hurtful to woody growth, and a similar result has 



* " Tharaiidter Jliibch.," vol. ix., 1853, p. 109; also vol. xxi., 1871, p. 218. 

 + Id., vol, xxii., p. 18r> ; vol. xxiii., p. 217. 

 X Id., 1888, p. 144 



§ llcuss," Ztschrft.fr. Frst. u. .Tgdwsn. , 1881," p. Cm. Also'- Cntilhltt, fr.d. ges 

 Frstw.," 1881, p. 2G7 ; id., 1882, p. 443. 



