"without fresh Supplies of Water and Air. 489 



their texture, so that a gentle touch caused their separation from 

 the footstalk ; and both exerted this injurious operation, when 

 present in such minute proportions as to be wholly inappreciable 

 by the animal senses. 



After having suffered much injury from these acid gases, the 

 plants, if removed in time, will recover, but with the loss of 

 their leaves. Hence, in vegetation carried on in a smoky at- 

 mosphere, the plants are rarely killed altogether, but merely 

 blighted for the season. Accordingly, in spring, vegetation 

 recommences with its accustomed luxuriance ; and as in many 

 situations there is, at that season and through the summer, a 

 considerable diminution in the number of coal fires, there will 

 be a proportionate decrease in the production of sulphurous 

 acid gas ; and, consequently, less injury will be done to plants 

 during that season. In winter, too, when coal fires mostly 

 abound, and gas is most abundantly generated, deciduous plants 

 are protected from its noxious operation by suspension of their 

 vegetating powers; but the leaves of evergreens, which continue 

 to grow through that season, are constantly exposed to its action, 

 when present in its greatest intensity. Accordingly, in many 

 of the suburban districts around London, especially in the 

 course of the river, where new manufactories are constantly 

 rising up, the atmosphere is so highly charged with noxious 

 matters, that many deciduous plants, and almost all evergreens, 

 cease to flourish, or exhibit only a sickly vegetation. 



In an interesting biographical sketch of his late lamented 

 friend Dr. Turner, Professor Christison confirms, by subsequent 

 experience, the opinion formerly given respecting the noxious 

 operation of the sulphurous and muriatic gases on plants : he 

 describes their action as so energetic, that, in the course of two 

 days, the whole vegetation of various species of plants may be 

 destroyed by quantities so minute as to be altogether inappreci- 

 able by the senses. On two occasions he was able to trace the 

 identical effects of the same kind of works (the black ash 

 manufactory) on the great scale, which his friend and himself 

 witnessed in their researches. In one instance, the devastation 

 committed was enormous, vegetation being for the most part 

 miserably stunted, or blasted altogether, to a distance of fully a 

 third of a mile from the works, in the prevailing direction of the 

 wind. Against the evils arising from such a vitiated atmosphere, 

 the plan of Mr. Ward provides effectual protection, as the 

 success of his own establishment amply demonstrates. 



5. Condition of Plants, in regard to Water, in close Cases and in 

 the free Atmosphere. 



In considering the conditions essential to vegetation, water 

 may be allowed to claim the first place ; for, if the vegetable 



