SULPHUROUS ACID. 79 



acid under the action of light, and it is the latter acid which is 

 injurious to plants. Agreeing with Von Heine, Wislizenus concludes 

 that sulphuric acid destroys the chlorophyll of the plant, and that it thus 

 acts in a manner unfavourable to assimilation. Sulphuric acid dries 

 the cell- walls and renders them impermeable. Only the tissues in 

 contact with the fibro-vascular bundles remain fresh and green, whilst 

 the others whither, brown, or bleach. In that lies the characteristic 

 symptom of these poisonings. According to Von Heine, the trans- 

 piration of the poisoned leaves, the cell-walls of which have become 

 impermeable, is less than that of healthy leaves. As the activity of 

 transpiration expresses the production of matter, the leaf assimilates 

 less, the plant remains stationary, and finally dies. The greater the 

 amount of sulphurous acid absorbed, the more intense the sun, the 

 greater the heat, and the drier the air, the more quickly are these 

 symptoms produced. All leaves, however, do not absorb the same 

 amount of sulphurous acid in the same atmosphere. With equal 

 leaf-surface, those of conifers absorb much less of this gas than those 

 of deciduous trees and herbaceous plants. It was believed that the 

 gas being absorbed by the stomata of the leaf, the quantity retained 

 would be proportional to their number, but experience has shown that 

 it is not so, for leaves absorb this gas quite as well by their upper 

 surface. The herbaceous plants are ranked, according to their sensi- 

 tiveness to sulphurous acid, between the conifers and the deciduous 

 trees. The leaves of the papilionacecB (pea tribe), potatoes, cereals, 

 and meadow grass, commence to fade and to brown at their extremities 

 after exposure of two hours in air containing 4(j|f(j()^ sulphurous acid ; 

 in an atmosphere containing only ^j^jj-^jj of this gas desiccation 

 does not occur until after fifteen to twenty hours. For leaves of 

 deciduous trees to perish the air must contain , woo *o .ijjIjjjj of sul- 

 phurous acid. Conifers in general, and especially fii'S, do not support 

 air containing more than lo^oo to Trjj^j^jj of this gas, especially in the 

 neighbourhood of factories. This small quantity of sulphurous acid 

 necessary to cause a pathological condition of the plant is the reason 

 why each factory chimney must be regarded as a perpetual hot-bed of 

 infection. The smoke from the combustion of coal contains on an 

 average O'Ol to 0-02 of sulphurous acid gas. The danger is obviated 

 if the smoke is sent into the air byjchimneys measuring 20 metres, say 

 65 feet, in height. The following, for example, shows the deterioration 

 undergone by a field of oats situated near a coke factory. One thou- 

 sand seeds of normal oats ought to weigh, on an average, 25-29 

 grammes ; 1000 grains in the sickly field only weighed 14*76 grammes. 

 Both the grain and the straw in this field were analysed, and in both 

 instances a large amount of sulphuric acid was found. Such damage 

 to crops is particularly intensified around chemical factories, chiefly 

 sulphuric acid factories, where pyrites is roasted, and zinc factories, 

 in which blende, zinc suljjhide, is calcined. The deadly action of the 

 sulphurous acid sent into the air by these factories manifests itself at 

 great distances. Forests 5 kilometres, say 3^ miles, away still suffer 

 from their proximity. A zinc works calcining 50 tons of blende dis- 

 engages 37,000 hectolitres (814,000 gallons) of sulphurous acid. It 



