XX INTRODUCTION. 



animals alone are injured by respiring air from which 

 oxygen has been abstracted by previous respiration : 

 change of air, whether beneficial or otherwise, does 

 take place, for our contrivances, although they 

 retard, cannot preclude a change. Thus the sup- 

 posed anomalies of plants hving without air, or 

 without change of air, are either dissipated or softened 

 down : we will inquire whence arise the benefits of 

 this plan. 



In London, the air is loaded with particles of soot, 

 than which there is scarcely any substance more 

 injurious to vegetation ; a single '' smut," as it is 

 usually called, causes a yellow mark wherever it has 

 adhered to a leaf; and the result of an atmosphere 

 loaded with smuts is the rapid destruction of the 

 leaves, so that the leaves of London trees are never 

 in a perfectly natural state ; they differ in appearance, 

 colour, and health, so to speak, from the leaves of 

 country trees : the deleterious effects of London 

 smut on the leaves influence the growth of the tree 

 itself, and London trees are invariably of slower 

 growth, and of less healthy appearance, than those in 

 the country. By the plan of cultivating plants in 

 closed vessels this injury is entirely avoided; the 

 smut and all solids borne by the atmosphere being 

 completely excluded, and forming a thick deposit on 

 the glass; if the vessel employed be a bell glass 

 inverted over the plant, then every accession of 

 atmospheric air must take place through the earth, 

 and consequently no portion of its impurities will be 



