194 BOARD OF AGRICULTURE. [Jan., 



very variable as to quantity, seldom more than ^^o-tli, or less than 

 gi^th of the bulk of the air. Water constitutes nearly | of the 

 weight of the plant, and from the surface of its leaves it is con- 

 tinually rising during the day. If the air was absolutely dry, it 

 would cause this evaporation to take place more rapidly than the 

 roots could supply it, and the whole plant would droop, wither, 

 and die. The animal organization is equally dependent upon wa- 

 ter, as it contains only about 40 lbs. of dry matter to 100 of water. 

 From the skin and lungs water is continually evaporating. ■ As in 

 the case of the plant, were the air perfectly dry, the skin would 

 parch and shrivel, the thirst would become intense, the air would 

 absorb the moisture as it was breathed out of the body, the ani- 

 mal would soon breathe away the fluids that filled out its tissues, 

 and become a ghastly mummy. Travelers tdl us of the great fa- 

 tality of the simoons and other hot winds of the desert which ap- 

 proach this condition of dryness. Again, without this moisture 

 in the air the heat of the earth would radiate into space and be 

 lost. We see also "that when the summer's sun has sunk beneath 

 the horizon, and coolness revisits the scorched plant and soil, the 

 grateful dew descends along with it, and moistens alike the green 

 leaf and thirsty land — the invisible moisture of the air thickens 

 into hazy mists, and settles in tiny pearls on every cool thing. 

 How thankful for this nightly dew has nature everywhere and 

 always appeared, and how have poets in every age sung of its 

 beauty and beneficence." From the same atmospheric store of 

 watery vapor come the refreshing rains, so we see that mmute as 

 it is in its division of particles, in the aggregate it is immense. 



We speak of the air as being pure, but it is scarcely ever per- 

 fectly so. Combustion, putrefaction, exhalations from marshes, 

 the fumes belched out of volcanoes, the accumulations of nitric 

 acid resulting from the discharge of hghtning; and in numerous 

 other ways, it is being constantly contaminated, and were it not 

 for the rains which constantly wash it and purify it, it would soon 

 become unfitted for animal life. 



We have thus spoken in as concise a m.anner as possible of the 

 chemical quahties of the air; let us now look briefly at the physi- 

 ology of its action upon the animal economy, as another prefatory 

 step to our subject. We are all of us reasonably familiar with the 

 anatomy of the lungs, the great organs of respiration. With the 



