312 JOUENAL OF THE EOYAL HOETICULTURAL SOCIETY. 



paper.* He gives two analyses of London fogs, one from Chelsea, the 

 other from Kew. Supposing a country fog (formed by condensation of 

 moisture in the air, far away from any smoke) to consist pretty nearly 

 of 100 per cent, of water, we find that a fog at Chelsea had 39 and at Kew 

 42-0 per cent, of carbon. Then, while the fog at Chelsea had over 

 14 per cent, of hydrocarbon, &c., Kew had nearly 5 per cent., showin.^^ 

 that there is by no means any uniformity in the contents of fogs. Sul- 

 phuric acid occurred in both to about 4 per cent. Mineral matters- 

 amounted to from 34 to nearly 42 per cent. Lastly, water was only 

 6 per cent. It is no misuse of terms, therefore, to call them " dry 

 fogs ! 



We have considered the effects of carbon and hydrocarbons as making 

 a tenacious grey coating upon glass, but the injury upon the plants by 

 direct contact is much greater. 



Although the analyses mention sulphuric and not sulphurous acid,, 

 the former is only derived from the latter by oxidisation ; and both acid?; 

 are highly injurious. Prof. Oliver found that the amount of sulphuric 

 acid in London air varied considerably, in his laboratory at University 

 College, Gower Street. Thus the average amount in dull weather was 

 6 milligrammes per 100 cubic feet of air. The amount in a slight fog 

 was 8'16 ; while in a thick yellow fog it amounted to 20*4. Sir William 

 Dyer points out that sulphuric acid (H^OSO;,) when formed becomes, 

 permanent and cumulative ; when diluted, it has no immediate effect on 

 wood ; but if it be heated so that water is driven off, then the SOjj at 

 once burns into the wood. Hence he says : " H^OSOg acts as a persistent 

 and gradual caustic. The same acts again and again and eats into the 

 tissues." 



On the other hand, sulphurous acid is mostly injurious to herbaceous, 

 and soft wooded plants. It penetrates the houses, and the water on the 

 glass sides, produced by evaporation, becomes charged with it, so that any 

 leaves which happen to press against the wet glass turn brown and the 

 portion dies. Or, again, as the water drips from the tips of leaves, the 

 tips turn brown and shrivel. As another effect, the water will not readily 

 evaporate ; vaporised tarry matters probably helping to cause this : the 

 result is that Fern-fronds hang down, get pressed together and rot. 



The way sulphurous acid acts on the plant is by abstracting oxygen 

 from the living protoplasm (being a powerful deoxidiser) in becoming 

 sulphuric acid. It thus, of course, kills the protoplasm and destroys the 

 necessary turgescence of cells for active growth. The consequence i.s 

 that leaves and flowers die and fall off. 



The general effect of the sulphurous acid, as well as the vaporised 

 and toxic-hydrocarbonaceous matters, is local discoloration of the leaves,, 

 and if the rest be still green they remain on. The local blotching is 

 actually due to sulphuric acid ; for on watering the plants the water takes 

 up the sulphurous acid, and then by oxidisation the sulphurous is changed 

 into sulphuric acid, which kills the protoplasm. On examining such 

 leaves microscopically Prof. Oliver observes that it is found that the 

 upper epidermis is first attacked. The acid traverses the cuticle and 

 destroys the underlying cells. Destruction of the protoplasm follows. 

 * Joiirn. Royal Hort. Soc. vol. xvi. p. 1, 1893. 



