50 



Prof. J. P. Cooke on Lecture Experiments. 



Fisr. 1 



The first fact, that HC1 consists of hydrogen and chlorine 

 gases, we prove by the electrolysis of strong liquid hydrochloric 

 acid, having previously shown in the course of the lecture how 

 HC1 is obtained from common salt, and that the liquid acid is 

 merely a solution of the gas in water. The decomposing cell 

 which we use in this experiment is represented in outline by 

 fig. 1, which is drawn to a 

 scale of one-fifth, so that 

 both its construction and 

 dimensions require no de- 

 tailed description. The 

 two small open glass cylin- 

 ders are fitted by grinding 

 with emery to the two tu- 

 bulatures of the cell, and 

 when the instrument is not 

 in use are closed above with 

 ground glass stoppers. 

 When mounted for use, 

 the cell is filled with strong 

 hydrochloric acid to with- 

 in an inch of the upper 

 mouths of the cylinders, and the glass stoppers are replaced by 

 rubber stoppers through which pass the gas-delivery-tubes and 

 the conducting-wires connecting with the electrodes, which are 

 two strips of platinum-foil 4 inches long by 1 inch wide. The 

 platinum conducting-wires are welded* to the strips of foil and 

 secured in the rubber stoppers as represented in detail by fig. 2. 

 Into the lower end of a short piece of glass tube, which passes 

 tightly through the rubber, the platinum wire connecting with 

 the foil is secured by melting the glass around it, while the 

 upper end is left open to receive the copper conducting-wires 

 from the battery, and the connexion is made perfect by placing 

 a few drops of mercury in the tube. This simple form of con- 

 necting-cup is easily made and very convenient. 



As the conducting-power of hydrochloric acid is very good, 

 three Bunsen's cells of the ordinary size have ample intensity 

 and give a rapid evolution of gas. The hydrogen gas is con- 

 ducted into a tall but narrow glass jar mounted in the usual 

 way over a small pneumatic trough, while the chlorine is col- 



* The platinum wire is very easily welded to the foil on a smoothed sur- 

 face of a lump of quicklime which serves as an anvil. The two, having been 

 placed together in the required position on this support, are first intensely 

 heated at the points to be united, with a blowpipe-name, and then a quick 

 blow with a small hammer completes the work. 



