110 THE EXTENSION OF THE PRINCIPLE. 
condition of the atmosphere, so essential to the 
well-being and even existence of such plants, we 
have it in our power to obtain in large towns ; 
and by warming and moistening the air we can, 
in fact, closely imitate any climate upon the face 
of the earth. It cannot be denied that in a pure 
and properly regulated atmosphere we possess a 
remedial means of the highest order for many of 
the ills that flesh is heir to ; and every medical 
man knows well, by painful experience, how nu- 
merous are the diseases which, setting at nought 
his skill and his remedies, would yield at once to 
the renovating influence of pure air. The diffi- 
culty to be overcome would be the removal or 
neutralization of the carbonic acid given out by 
animals ; but this in the present state of science 
could easily be effected, either by ventilators, 
or by the growth of plants in connexion with the 
air of the room, so that the animal and vegetable 
respirations might counterbalance each other. 
The volume of the air, with the quantity of ve- 
getable matter required, as compared with the 
size and rank of the animal in Creation, would 
be a problem well worthy of solution. Experi- 
ments of this kind upon any scale might be 
instituted in the Zoological Gardens, where the 
moping owl and ivy-mantled tower might be as- 
sociated. In one of my own houses, about ten 
