GASEOUS DIFFUSION. Ill 



suffocated on inhaling the "fixed air," because this gas cannot 

 carry on the action in the lungs essential to life, which action 

 requires the presence of sufficient free oxygen. Many fatal 

 accidents happen yearly on account of this tendency, and the 

 neglect of simple precautions to ascertain whether the air in a well 

 or cave, &c., is dangerous to breathe or safe. Of these precautions 

 the simplest is to lower down a lighted candle or lantern before 

 making a descent ; if the light continues burning, the air is not 

 so impure as to be dangerous on account of the liability to suffoca- 

 tion; but if the light become extinguished, the atmosphere is 

 unsafe to breathe, and any one descending will probably be 

 suffocated for want of oxygen, just as the candle flame goes out 

 on account of the same want. 



When explosions take place in collieries, the air in the under- 

 ground passages becomes irrespirable, because the oxygen present 

 is consumed in burning the inflammable gas or " fire-damp " that 

 exudes from the coal, so that a mixture of nitrogen and carbon 

 dioxide with little or no free oxygen is produced : this suffocating 

 gas is termed the after-damp, and is frequently more dangerous to 

 life than the explosion itself. 



Expt. 103. Diffusion of Carbon Dioxide into the Air. 

 Pass the delivery tube of a carbon dioxide generator to the 

 bottom of a wide tumbler or pudding basin, so as to fill it with gas ; 

 a lighted candle lowered into the basin will be extinguished, as in 

 Expt. 101. Set the basin aside on a shelf in a cupboard for twenty- 

 four hours, and then test it again with a lighted candle ; the candle 

 will now burn freely; showing that the heavy carbon dioxide has 

 diffused away, and the lighter air taken its place. If a very tall 

 narrow cylinder be used instead of a tolerably wide shallow vessel, 

 a longer time than twenty-four hours will probably be requisite 

 before the carbon dioxide has diffused away sufficiently to enable 

 a candle to burn. 



Expt. 104. To produce Hydrogen Gas. Place some granu- 

 lated zinc (Expt. 16) in the gas generator instead of chalk (after 

 having carefully cleaned out the bottle), and then pour diluted 

 hydrochloric acid down the funnel, so as to cause hydrogen to be 

 evolved (Expt. 10). Arrange the delivery tube so as to point 

 upwards, connecting it with the generator by means of a piece of 

 flexible tubing. Hydrogen, being lighter than air, may then be 

 collected by placing a wide-mouthed jar bottom upwards over the 

 issuing jet of gas (fig. 59). 



Expt. 105. To illustrate the Lightness of Hydrogen. 

 Having collected a jar of hydrogen, place it on the table mouth 

 downwards. Hold the jar in one hand, and a lighted taper in the 



