1883.] THE VENTILATION OF FARM BUILDINGS. 193 



ous diffusion." This is a property that all gases have of inter- 

 mingling one with another, as wine and water will intermingle. 

 Many interesting experiments could be given illustrating this law, 

 and showing how rapidly this diffusion takes place ; so accurately 

 is this known that chemists have learned to compute it with mathe- 

 matical exactness. It is a law that is continually performing an 

 important work in the atmosphere around us; accumulations of 

 gases which are unfitted for the support of life, are by this means 

 silently and effectually dispersed, and thus the composition of the 

 aerial ocean which surrounds us is uniformly maintained. Inde- 

 pendent of this we have winds and aerial currents which tend to 

 blend and mix together the various gases of which the air 

 consists. 



Of the purposes served by these several constituents of the air, 

 we will glance one moment. Oxygen is as necessary to the ani- 

 mal economy ^ any portion of its food, and it can only obtain it 

 from the air; every breath that is drawn supplies a portion, and a 

 like supply is necessary every moment. It is also necessary for 

 combustion; without it fuel would not burn, and we could have 

 neither light nor heat. If it was pure it would exhaust the body 

 too soon, and life would be of brief duration, and combustion 

 could not be controll^ ; but it is admirably adapted to the exist- 

 ing order of life, by being properly diluted with nitrogen; this 

 not being poisonous as carbonic acid is, counterbalances the too 

 active oxygen ; it weakens and prolongs its action, and assuages its ■ 

 fiery influence. 



But what of the deadly carbonic acid gas ? It is as necessary 

 to vegetable life as oxygen is to animal life. Every green leaf of 

 the field and forest absorbs it throughout the day, breathing out 

 in its stead oxygen, the reverse of animal respiration and were 

 it not present in our air, the earth would be deprived of plant 

 life, and consequently all animal life, for the animal is dependent 

 upon the vegetable. The small amount of this gas present in or- 

 dinary air will allow of animals breathing it with impunity, but 

 when increased beyond a certain amount, as we shall in time show, 

 respiration becomes impossible; even though the amount of oxygen, 

 could be increased, the poison of carbonic acid would cause death 

 should it rise above a certain "point. 



So also the watery vapor performs an important function. It is 



13 



