RESPIRATION. 99 



ration of air, in a greater or less degree, is constantly neces- 

 sary to the existence of every living tiling. 



' The presence of air is as necessary to the life of plants, 

 as to that of animals. They have a respiration carried on by 

 means of their leaves, which consumes in the same way the 

 oxygen of the atmosphere, and exhales, instead of it, carbonic 

 acid.' 



When placed in an exhausted receiver, the air contained 

 in every part of their substance is soon extracted ; and, in pro- 

 portion as this air is likewise pumped out by the machine, the 

 flowers and leaves show evident symptoms of debility ; they 

 become flaccid, pendulous, 'and assume a sickly appearance; 

 and, if retained in that situation a certain length of time, their 

 vegetating powers are irrecoverably extinguished. 



Upon the whole, as the air we continually breathe is liable 

 to be impregnated with exhalations from every substance to 

 which it has access, the great importance of attention to its 

 purity is an obvious reflection. In building towns or houses, 

 the situation, with regard to air, is a capital object. The vi- 

 cinity of marshes, of stagnating waters, of manufactures of 

 tallow, oil, sal-ammoniac, the smelting or corroding of metals 

 of every kind, and many other operations which contaminate 

 the air, should be either avoided or removed, as they are the 

 pests of our senses, and the poisoners of our constitutions. 

 Even in northern climates, houses surrounded with trees, or 

 in the neighborhood of luxuriant vegetables, are always damp, 

 and infected with insects; and hence the ambient air is re- 

 plete with the seeds of disease. Precautions of this kind are 

 still more necessary in hot climates. Air absorbs a greater 

 or less proportion of the particles of bodies, according to its 

 degree of heat. In Madrid, however, in Constantinople, and 

 in many other cities of warm regions, the houses are crowded 

 together, the streets are narrow, and covered with tilth of 

 every kind. We cannot, therefore, be surprised, that human 

 beings, existing in such situations, should be so frequently in- 

 fected with pestilential diseases. 



