OXYGEN AND ITS SALINE COMBINATIONS 



167 



an electric spark, because the spark heats the space through which it 

 passes, and acts consequently in a manner similar to ignition by means 

 of contact with an incandescent or burning substance. In fact, instead 

 of a spark a fine wire simply 

 may be taken, and an elec- 

 tric current passed through 

 it to bring it to a state of 

 incandescence ; in this case 

 there will be no sparks, but 

 the gases will inflame if the 

 wire be fine enough to be- 

 come red hot by the passage 

 of the current. Cavendish 

 made this experiment on the 

 ignition of detonating gas, 

 at the end of the last cen- 

 tury, in the apparatus shown 

 in fig. 31. Ignition by the 

 aid of the electric spark is 

 convenient, for the reason 

 that it may then be brought 

 about in a closed vessel, 

 and hence chemists still em- 

 ploy this method when it is FIG. 31. Cavendish's apparatus for exploding detonatin 

 . . . gas. The bell jar standing in the bath is filled wit 



required to ignite a mixture 

 of oxygen with a combus- 

 tible gas in a closed vessel. 

 For this purpose they now, 

 especially since Bunsen's 

 time, 30 employ an eudiometer. 



It consists of a thick glass tube graduated along its length in milli- 

 metres (for indicating the height of the mercury column), and 

 calibrated for a definite volume (weight of mercury). Two plati- 

 num wires are fused into the upper closed end of the tube, as 

 shown in fig. 32. They must be hermetically sealed into the tube, 

 so that there be no aperture left between them and the glass. 31 



50 Now, a great many other different forms of apparatus, sometimes designed for 

 special purposes, are employed in the laboratory for the investigation of gases. Detailed 

 descriptions of the methods of gas analysis, and of the apparatus employed, must be 

 looked for in works on analytical and applied chemistry. 



31 In order to test this, the eudiometer is filled with mercury, and its open end 

 inverted into mercury. If there be the smallest orifice at the wires, the external air will 

 enter into the cylinder and the mercury will fall, although not rapidly if the orifice 

 be very fine. 



a mixture of two volumes of hydrogen and one volume of 

 oxygen, and the thick glass vessel A is then screwed 

 into it. The air is first pumped out of this vessel, so 

 that when the stop-cock c is opened, it becomes filled 

 with detonating gas. The stop cock is then re-closed, 

 and the explosion produced by means of a spark from 

 a Leyden jar. After the explosion has taken place the 

 stop-cock is again opened, and the water rises into the 

 vessel A. 



