76 EXPERIMENTAL GENERAL SCIENCE 



a one-hole stopper through which the glass tube is thrust. The 

 oxygen given off through this tube, if conducted into a dish of 

 water, may be seen bubbling up through it. By filling the test- 

 tube with water and inverting it in the dish of water over the 

 rising bubbles of gas, they will displace the liquid in the test- 

 tube. Before being removed from the water the test-tube 

 must be closed with the finger or a small sheet of glass to pre- 

 vent the escape of the gas. If a large bottle or other recep- 

 tacle be filled with the gas it may also be prevented from escap- 

 ing by covering the mouth with a sheet of glass. Since all 

 gases are lighter than water, all may be caught over water 

 in the same way. Hydrogen is so much lighter than air that it 

 may also be caught over air in an inverted test-tube. Carbon 

 dioxide is best prepared by pouring a little dilute hydrochloric 

 acid over a few pieces of limestone or marble in a test-tube. It 

 may also be obtained by heating baking soda. 



73. Carbon Dioxide. The union of carbon with oxygen 

 forms a colorless, odorless, tasteless gas called carbon dioxide. 

 Since the oxygen it contains is not in a form that our bodies can 

 use in respiration, carbon dioxide is a suffocating gas, but it is 

 not poisonous as is frequently supposed. It is this gas that 

 gives the sparkle to wine and other effervescent drinks and the 

 pungent taste to soda water. Bread, cake, and the like are 

 made light by bubbles of carbon dioxide in the dough. Al- 

 though carbon dioxide does not support ordinary combustion, 

 the metal magnesium will burn in it, tearing the oxygen from 

 the carbon for the purpose. When carbon dioxide is mixed 

 with lime-water, it gives the latter a milky appearance, and 

 this liquid is commonly used as a test for the gas. Carbon 

 dioxide is slightly heavier than air and has a tendency to 

 accumulate in the bottom of wells, abandoned mines, caves, 

 and similar places. People are sometimes suffocated by 

 venturing into such places without first testing it for this gas. 

 Since fires will not burn in carbon dioxide, another test for 



