THE THIRD CONSTITUENT OF THE AIR 23 



taper. Note that the escape of the gas from the bottle is very 

 slow, for it remains at the bottom like a liquid. Try to pour 

 it from one bottle to another like water. Pour some water 

 into a cylinder of the gas, and, having covered with a greased 

 plate, shake vigorously. Now attempt to remove the plate, 

 and note whether there is evidence of the gas being soluble. 

 Test the liquid in the bottle with litmus solution, and note 

 whether it is acid. These experiments show that the gas is 

 unable to support combustion, heavier than air, and soluble 

 in water forming an acid solution. 



Air and Ventilation. — This oxide of carbon, usually known as 

 carbonic acid gas, is a product of combustion, not only of charcoal, but of 

 all ordinary fuels, solid, liquid, and gaseous, all of which contain combined 

 carbon. This may be shown by burning a candle, a jet of coal gas, etc. 

 in a bottle, closed at the top with a brass plate, until the flame expires, 

 and then testing the contents of the bottle with lime water. Carbonic acid 

 gas is also a product of animal respiration. If a jar of air expired from the 

 lungs be collected over water, it will be found to render lime water milky, 

 and to • be so rich in carbonic acid gas and deficient in oxygen as to 

 extinguish a lighted taper. The amount of carbonic acid gas normal!}- 

 present in air is not more than .03 or .04 per cent., and this small amount 

 is not injurious to health. In the air of towns it may reach .06 per cent., 

 and quantities in excess of this are held to be prejudicial to health. In 

 London air . i per cent, is sometimes observed, while in a crowded room it 

 may reach .3 per cent. Air containing 3 per cent, extinguishes a lighted 

 taper, while air containing 4 per cent, is fatal to life. 



An adult breathes out about .6 cubic feet of carbonic acid gas per hour, 

 while a jet of coal gas burning at the rate of 3 cubic feet per hour produces at 

 least 1.4 cubic feet of carbonic acid gas. The carbonic acid gas in the air 

 cf a room having a capacity of 10,000 cubic feet, would therefore in one 

 hour increase from .04 to .06 per cent, (the healthy limit) by the respira- 

 tion of one man and combustion of one gas jet, if there were no ventila- 

 tion. As vitality is reduced by breathing vitiated air, and the germs of 

 disease flourish in a close and heated atmosphere, the efficient ventilation 

 of dwelling-rooms and public buildings, as well as of sheds where Airm 

 stock are housed, is a matter of very great importance ; and many instances 

 are recorded of the spread of disease among human beings and among 

 cattle owing to want of ventilation. 



Ventilation is effected by utilising three principles :—(i) Gases difTiisc 



