40 CHEMISTRY FOR AGRICULTURAL STUDENTS 



hydrogen = i, the relative density of air = 14.4. Hydrogen, being the 

 lightest gas known, is usually taken as unity. 



When heated, gases expand much more than liquids or solids, and, unlike 

 liquids and solids, they all expand equally and also regularly for each 

 increment of temperature. On heating from 0° to l° C, a gas expands 

 T>fj of its volume, from i° to 2" -^, and so on. Thus 273 c.c. of air at 0° 

 become 288 c.c. at 15°; 288 c.c. at 15° become 283 c.c. at 10°. The 

 necessity for knowing the temperature in calculating the weight of air, and 

 for having the hydrogen and air at the same temperature in the foregoing 

 experiment, will now be understood ; and should the temperature not be at 

 15°, it will be possible to calculate the weight of air in the flask 

 at any other temperature. 



Gases are far more elastic and compressible than liquids 

 and solids. The volume of any gas varies inversely with the 

 pressure put upon it. Thus looo c.c. of air under a pressure 

 of 30 inches of mercury become 1500 c.c. under 20 inches. 

 This may be illustrated by means of the tube figured. If a 

 little mercury at the bottom of the tube stand at the same level 

 in both limbs, the air in the closed hmb will be at the atmos- 

 pheric pressure (say 30 inches of mercury). If now mercury be 

 poured into the tube until it stands 30 inches higher in the 

 long limb than in the short, the pressure upon the air in the 

 short limb will be doubled, and the volume will be found to be 

 reduced to half. If another 30 inches of mercury be added the 

 pressure will be trebled, and the volume of gas reduced to one- 

 third. In the foregoing experiment it is obviously necessary to 

 take the barometric pressure into account, and should the 

 barometer not stand at 30 inches, it is possible to calculate the 



Fig. 23. weight of 1000 c.c. of air under any other pressure. 



In what proportion does hydrogen unite with oxygen to form 

 water? This may be determined by finding the quantity of 

 water formed by the action of hydrogen on a known weight of 

 heated oxide of copper, when the following change occurs : — 



Hydrogen -f oxide of copper = copper -f oxide of 

 hydrogen. 



Fit up the apparatus figured. In the hard glass tube E 

 place some oxide of copper. Heat the tube to a low red heat, 

 and draw a current of air through the apparatus till every trace 

 of moisture has been driven off; allow to cool ; plug the holes 



