80 Domestic Science 



from the end of the tube, leaving a plug about |" thick. 

 Cover this end of the tube, clamped in an upright 

 position, with a large test-tube. Pass a piece of narrow 

 indiarubber tubing, connected to the gas-supply, up the 

 tube till the end of the rubber tube is nearly in contact 

 with the plaster plug. Turn the gas-cock full on and 

 allow the gas to pass for half a minute. Very slowly 

 withdraw the rubber tube, and, when the open end of 

 it reaches the lower end of the glass tube, close this 

 end of the latter with the thumb. Turn off the gas 

 and lower the end of the glass tube, still closed by the 

 thumb, into a Vessel of water coloured with litmus. 

 Remove the thumb under water and clamp the glass 

 tube in a vertical position with its lower end about J" 

 below the surface of the water. Remove the test-tube 

 from the upper end. After a few minutes the water 

 will be found to have risen in the tube above the level 

 of that outside the tube and this rise will continue for 

 some time, the water inside the tube sometimes reaching 

 a point several inches above the level of the water in 

 the outer vessel. Then a gradual fall occurs until the 

 water-levels inside and outside the tube are the same. 

 As far as the eye can judge, the plaster of Paris 

 plug is a continuous solid substance, but it really 

 consists of innumerable very small particles separated 

 by minute channels. The molecules of the constituents 

 of the coal-gas inside the tube and of the air outside 

 can traverse these channels readily, and do so, but at 

 different rates. The molecules of the less dense con- 

 stituents of the coal-gas escape from the tube through 

 the plug much more rapidly than the air molecules 

 pass inward. The result is that the total quantity of 

 gaseous matter inside the tube is lessened and its 

 pressure on the water-surface (see Chapter vi) decreased 



