XXll 
INTRODUCTION. 
gentle touch caused their separation from the footstalk ; and 
both exerted this injurious operation when present in such 
minute proportions as to he wholly inappreciable to the 
animal senses. After having suffered much injury from 
these acid gases, the plants, if removed in time, will re- 
cover, hut with the loss of their leaves. Hence in vegeta- 
tion, earned on in a smoky atmosphere, the plants are 
rarely killed altogether, but merely bhghted for the season : 
accordingly, in spring vegetation re^^ommences with its ac- 
customed 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 suspen- 
sion of their vegetating powers ; hut the leaves of evergreens, 
which continue to grow during 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 decidu- 
ous plants, and almost all evergreens, cease to flourish, or 
exhibit only a sickly vegetation. In an interesting bio- 
graphical sketch of his late lamented friend Dr. Turner, 
Professor Christison confirms, by subsequent experience, 
the opinion formerly given respecting the noxious operation 
