THE AIR OF TOWNS. 353 



turbid indicated the presence of carbonic acid.) It is given off from the 

 breath, as the following experiment will show: 



By filling a bell jar (fig. 5) with water and breathing air into it from 

 the lungs an atmosphere is obtained within the jar which readily extin- 

 guishes a taper, indicating the large percentage (about 5 per cent) of 

 carbonic acid in the breath. 



Two bottles (fig. 6), each provided with a double neck, are so con- 

 nected that air may be drawn into the lungs through the liquid 

 contained in A and expelled through the liquid in B without removing 

 the tube from the mouth. If clear limewater is introduced into these 

 two vessels, that contained in B will very shortly become turbid, indi- 

 cating the presence of carbonic acid in the lungs, whilst A remains clear. 



Carbonic acid is produced by fermentation and the decay, which is 

 another form of fermentation, of animal and vegetable substances. 



A solution of grape sugar is introduced into a flask (fig. 7), together 

 with a quantity of brewers' yeast. The flask is provided with a cork 

 through which a bent tube passes. The longer limb dips into a test 

 glass containing limewater. If the flask is allowed to stand at the 

 ordinary temperature, the liquid begins to froth and bubbles of car- 

 bonic acid rise through the limewater, turning it milky. After a few 

 hours a sufficient quantity of alcohol will be formed to enable its 

 presence to be demonstrated. On bringing some of the liquid into a 

 flask fitted with a long glass tube and boiling it, the vapors passing 

 out of the tube will take fire and burn with the blue flame of burning- 

 alcohol. 



All these processes go on at the expense of the oxygen of the air, 

 which in time would disappear. It has been estimated that it would 

 require 900,000 years to consume all the oxygen in the air and convert 

 it into carbonic acid. Long before this, however, life would have ceased 

 on the earth, for a slight increase in the amount of carbonic acid or 

 diminution of oxygen would render the atmosphere unfit for respiration. 



We are fortunately not threatened by any such catastrophe. No 

 accumulation of carbonic acid can occur in the open air under natural 

 conditions, for although carbonic acid is a heavy gas, it rapidly 

 diffuses. 



Two flasks (fig. 8) are connected by a long piece of narrow tube. In 

 the lower flask the heavy gas, carbonic acid, is introduced, and in the 

 upper one, the light gas, hydrogen. Owing to the property of dif- 

 fusion some of the heavier gas will be found after a time to have passed 

 into the upper flask and the lighter gas to have passed downward. 



Carbonic acid therefore becomes quickly disseminated through the 

 atmosphere. Vegetation now steps in. The green coloring matter of 

 plants, termed chlorophyll, has the property in presence of sunlight of 

 splitting up the carbonic acid, absorbed from the air around, into carbon, 

 which it retains for its own growth, and into oxygen, which is restored 

 to the atmosphere. We need not, therefore, trouble ourselves with the 

 SM 95 23 



