26 THE FOKMS OF WATER IN 



Simply turn the cock and allow the compressed &ir to 

 stream into the atmosphere. The current, if allowed to 

 strike a thermometer, visibly chills it; and with othei 

 instruments the chill may be made more evident still. 

 Even the hand feels the chill of the expanding air. 



77. Throw a strong light, a concentrated sunbeam for 

 example, across the issuing current ; if the compressed 

 air be ordinary humid air, you see the precipitation of a 

 little cloud by the chill accompanying the expansion. 

 This cloud-formation may, however, be better illustrated 

 in the following way : — 



78. In a darkened room send a strong beam of light 

 through a glass tube three feet long and three inches 

 wide, stopped at its ends by glass plates. Connect the 

 tube by means of a stopcock with a vessel of about one- 

 fourth its capacity, from which the air has been re- 

 moved by an air-pump. The exhausted cylinder of the 

 pump itself will answer capitally. Fill the glass tube 

 with humid air ; then simply turn on the stopcock 

 which connects it with the exhausted vessel. Having 

 more room the air expands, cold accompanies the ex- 

 pansion, and, as a consequence, a dense and brilliant 

 cloud immediately fills the tube. If the experiment be 

 made for yourself alone you may see the cloud in ordi- 

 nary daylight; indeed, the brisk exhaustion of any 

 receiver filled with humid air is known to produce this 

 condensation. 



79. Other vapours than that of water may be thus 



