148 



PRINCIPLES OF CHEMISTRY 



water contains 12 '575 parts of hydrogen per 100 parts oxygen, that is 

 7-990 parts of oxygen per 1 part of hydrogen, and therefore it is usually 



with the copper oxide is weighed before and after the experiment. The loss in weight, 

 shows the quantity of oxygen which went into the composition of the water formed, 



the weight of the latter being 

 shown by the gain in weight of 

 the absorbing apparatus. Know- 

 ing the amount of oxygen in the 

 water formed, we also know the 

 quantity of hydrogen contained 

 in it, and consequently we deter- 

 mine the composition of water by 

 weight. This is the essence of the 

 determination. We will now turn 

 to particulars. In one neck of the 

 three-necked bottle there is placed 

 a tube immersed in mercury. This 

 serves as a safety-valve to pre- 

 vent the pressure inside the ap- 

 paratus becoming too great from 

 the rapid evolution of hydrogen. 

 Did the pressure rise to any con- 

 siderable extent, the current of 

 gases and vapours would be very 

 rapid, and, as a consequence, the 

 hydrogen would not be perfectly 

 purified, or the water be entirely 

 absorbed in the tubes placed for 

 this purpose. In the third neck 

 of the Woulfe's bottle there is a 

 tube leading the hydrogen to the 

 purifying apparatus, consisting 

 of eight U tubes, destined for the 

 purification and testing of the hy- 

 drogen. The hydrogen, evolved 

 by zinc and sulphuric acid, is 

 purified by passing it first through 

 a tube full of pieces of glass moist- 

 ened with a solution of lead ni- 

 trate, next through silver sul- 

 phate; the lead nitrate retains 

 sulphuretted hydrogen, and ar- 

 seniuretted hydrogen is retained 

 by the tube with silver sulphate. 

 Caustic potash in the next U tube 

 retains any acid which might 

 come over. The two follow- 

 ing tubes are filled with lumps of 

 dry caustic potash in order to ab- 

 sorb any carbonic anhydride and 

 moisture which the hydrogen 

 might contain. The next two tubes 

 are, to completely dry the gas, 

 filled with a powder of phosphoric 



