XX11 INTRODUCTION. 



air, manifested its action in a few hours,, and entirely 

 destroyed the plant in two days. Both these gases 

 acted on the leaves, affecting more or less their 

 colour, and withering and crisping 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 to 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 atmosphere, the plants are rarely killed 

 altogether, but merely blighted for the season : ac- 

 cordingly, 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 produc- 

 tion 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 suspen- 

 sion 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, espe- 

 cially in the course of the river, where new manu- 

 factories are constantly rising up, the atmosphere is 



