XX 
INTRODUCTION. 
injured by respiring air from which oxygen has been ab- 
stracted 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 supposed anomalies of plants living 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 difier 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 at- 
mospheric air must take place through the earth, and 
consequently no portion of its impurities will be deposited 
on the plant. Mr. Ward is perfectly right, when he attri- 
butes the sickly state of London vegetation to the de- 
pressing influence of the fuliginous matter with which the 
