Prof. J. P. Cooke on Lecture Experiments. 53 



evolved together from a simple decomposing-cell, shown more 

 in detail in fig. 9, but in connexion with another apparatus ; and 

 the connexion of the battery with the two electrodes is here 

 made as before shown in fig. 2. The mixed gases pass through 

 a small drying- tube filled with pumice moistened with oil of 

 vitriol into an absorption-tube whose construction is shown in 

 detail by fig. 6. As the gaseous mixture is decidedly lighter 

 than air, it is conducted to the top of the absorption-tube through 

 the small quill-tube, and overflows through the short tube 

 below. Both of these should pass air-tight through the rubber 

 stopper which closes the mouth of the large absorption-tube, and 

 which should therefore have a bearing on the glass of at least 

 half an inch. It will be noticed that, while the overflow- tube 

 is straight, the delivery-tube is bent so that its open end is four 

 or five inches above the mouth of the absorption-tube. Rubber 

 stoppers, or rather caps, must also be provided, by which the 

 open ends both of the delivery- and the overflow-tubes may be 

 closed at the right time. These are made by boring with a cork- 

 borer nearly through an ordinary stopper, and then pulling out 

 the rubber core with a pair of pincers. The absorption-tube 

 should now be divided into two parts of equal capacity by means 

 of rubber rings, having, however, a length of three or four inches 

 at the lower end for the absorbent liquid. 



The absorption-tube having been prepared as described and 

 filled with the mixed gases, the experiment is made in the fol- 

 lowing way : — The apparatus is first rapidly depressed in a solu- 

 tion of caustic soda (of the ordinary strength used in the labo- 

 ratory) until the liquid rises in the tube to the level of the first 

 ring. The open end of the delivery-tube is now quickly closed 

 with the rubber cap, and then the end of the overflow- tube is 

 closed in the same way, the second rubber cap having been pre- 

 viously dropped upon the bottom of the glass vessel containing 

 the soda solution so as to be ready for the purpose. The tube, 

 now perfectly air-tight, is removed from the liquid, and the solu- 

 tion of soda turned backward and forward through its entire 

 length until the chlorine is wholly absorbed. The end of the 

 overflow-tube is now opened under water coloured blue with 

 litmus, which, as it rushes in, will be instantaneously bleached, 

 indicating the presence of chlorine. The tube must now be 

 immersed in the water until the level of the liquid is the same 

 within and without, when it will be found that just one-half of 

 the gas has been absorbed. We have now proved that one-half 

 of the volume of gas evolved during the electrolysis of hydro- 

 chloric acid consists of chlorine, and it only remains to show 

 that the remaining half still left in the tube is hydrogen. For 

 this purpose the delivery-tube may be connected with a water-* 



