1888.] 



Densities of Hydrogen and Oxygen. 



359 



stand an internal vacuum. To guard more thoroughly against the 

 penetration of external air, every cemented joint was completely 

 covered with vaseline, and the vaseline again with water. Again, the 

 zincs were in the form of solid sheets, closely surrounding the plati- 

 nised plate on which the hydrogen was liberated, and standing in 

 mercury. It was found far better to work these cells by their own 

 electromotive force, without stimulation by an external battery. If 

 the plates are close, and the contact wires thick, the evolution of gas 

 may be made more rapid than is necessary, or indeed desirable. 



Tubes, closed by drowned stopcocks, are provided, in order to allow 

 the acid to be renewed without breaking joints ; but one charge is 

 sufficient for a set of experiments (three to five fillings), and during 

 the whole of the time occupied (10 to 14 days) there is no access of 

 atmospheric air. The removal of dissolved air (and other volatile 

 impurity) proved, however, not to be so- easy as had been expected, 

 even when assisted by repeated exhaustions, with intermittent evolu- 

 tion of hydrogen ; and the results often showed a progressive improve- 

 ment in the hydrogen, even after a somewhat prolonged preliminary 

 treatment. In subsequent experiments greater precautions will be 

 taken.* Experience showed that good hydrogen could not thus be ob- 

 tained from zinc and ordinary " pure " sulphuric acid, or phosphoric 

 acid, without the aid of purifying agents. The best results so far have 

 been from sulphuric and hydrochloric acid, when the gas is passed 

 in succession over liquid potash, through powdered corrosive subli- 

 mate, and then through powdered caustic potash. All the joints of 

 the purifying tubes are connected by fusion, and a tap separates the 

 damp from the dry side of the apparatus. The latter includes a 

 large and long tube charged with phosphoric anhydride, a cotton wool 

 filter, a blow-off tube sealed with mercury until the filling is com- 

 pleted, besides the globe itself and the Toppler pump. A detailed 

 description is postponed until the experiments are complete. It may 

 be sufficient to mention that there is but one india-rubber connexion, 

 — that between the globe and the rest of the apparatus, and that the 

 leakage through this was usually measured by the Toppler before 

 commencing a filling or an evacuation. 



The object of giving a considerable capacity to the phosphoric 

 tube was to provide against the danger of a too rapid passage of gas 

 through the purifying tubes at the commencement of a filling. Sup- 

 pose the gas to be blowing off, all the apparatus except the globe 

 (and the Toppler) being at a pressure somewhat above the atmo- 

 spheric. The tap between the damp and dry sides is then closed, 

 and that into the globe is opened. The gas which now enters some- 



* Spectrum analysis appears to be incapable of indicating the presence of com- 

 paratively large quantities of nitrogen. 



