ON EUDIOMETERS. 289 



fear of its falling down. The next thing is to introduce 

 the ball of potash, and it requires a little delicate manipu- 

 lation to get it into its proper position without the admis- 

 sion of any atmospheric air. The potash must not be 

 too hard. The bullet is placed below the mercury, 

 and by means of the fingers the mercury is carefully 

 rubbed against it so as to remove all traces of atmospheric 

 air. It is then introduced into the eudiometer, and when 

 it is in its proper position the end of the wire must be 

 left below the surface of the mercury in order to avoid any 

 possibility of any air being carried up by capillarity, 

 between the mercury and the platinum. You notice how 

 slowly the absorption is taking place, and this is pure 

 carbonic anhydride, so that it is in its best condition to be 

 absorbed, as there is always a supply in contact with the 

 potash. When you are determining by means of caustic 

 potash the quantity of carbonic anhydride which may be in a 

 gas, it is necessary to leave the bullet in contact for several 

 hours. When the absorption is complete the bullet is 

 removed by a slight jerk, so that any gas carried down by 

 it is left behind and not brought through the mercury. 

 Then the absorption tube is placed perfectly vertical and 

 left in a room by itself for at least half an hour, with a 

 thermometer standing by its side. Then the operator comes 

 into the room and stealthily approaching the cathetometer, 

 so as not to produce currents of air, reads off the tempera- 

 ture of the thermometer and the height of the column of 

 mercury. Having got this height he may more leisurely 

 measure the height of the column of mercury in the trough 

 on the outside of the tube. The tube being graduated 

 almost down to the mouth, you can measure in this way the 

 height of the column of mercury which it has in it, which 

 has to be deducted from the barometer in the calculations. 

 In all these experiments you must be very careful in 

 reading the thermometer and also the barometer. The 

 one used by Bunsen is a syphon barometer, graduated cr 

 one limb, and at the upper portion, it is divided from a zero 

 point in the centre, and the column of mercury is deter- 

 mined by adding together the two readings on the tubes. 

 Sometimes the zero point is placed below, and then yon 

 have to subtract the shorter column from the longer, but 



VOL. II. U 



