INTRODUCTION. XXI 



deposited on the plant. Mr. Ward is perfectly right, 

 when he attributes the sickly state of London vege- 

 tation to '' the depressing influence of the fuliginous 

 matter with which the atmosphere in which he lives 

 is surrounded :" but it appears that other causes have 

 been sought in the presence of gases injurious to 

 vegetable life. This theory I shall now examine. 



Mr. Ellis, in an excellent paper read to the Botanical 

 Society in June, 1839, and since pubhshed in the 

 Gardener's Magazine for September,* objects to the 

 idea previously expressed by Mr. Ward, of the dele- 

 terious influence of this smut or fuliginous matter; 

 and goes on to explain at length, that ^' the real mode 

 in which such an atmosphere proves injurious to 

 vegetation was first shown by the experiments of 

 Drs. Turner and Christison, which were published in 

 the ninety-third number of the Edinburgh Medical 

 and Surgical Journal. They ascertained that it is 

 not simply to the difllision of fuliginous matter 

 through the air, but to the presence of sulphurous 

 acid gas, generated in the combustion of coal, that 

 the mischief is to be ascribed. When added to com- 

 mon air, in the proportion of i or ^im part, that gas 

 sensibly affected the leaves of growing plants in ten 

 or twelve hours, and killed them in forty-eight hours 

 or less. The effects of hydro-chloric, or muriatic 

 acid gas, were still more powerful, it being found that 

 the tenth part of a cubic inch in 20,000 volumes of 



* The Gardener's Magazine, conducted by J. C. Loudon, vol. xv. 

 V. 488. 



