472 Bev. P. J. Kirkby on the Union of 



numbers in Table I. Nevertheless, the values of R prove 

 that the temperature of the platinum wire, at which the 

 catalytic action starts, is practically constant for the range of 

 pressures indicated by the tables. In fact, if the numbers 6 

 are affected with a large percentage error, this error is at 

 least a constant one. 



To ascertain whether the combination of the hydrogen and 

 oxygen was due to the temperature rather than to the 

 substance of the platinum, a wire of pure silver was 

 substituted for the platinum wire in an apparatus otherwise 

 precisely similar to that illustrated in fig. 1. The diameter 

 of the wire was '1 mm. It was coiled into a spiral ; and it 

 was set up in the tube as the platinum wire had been by means 

 of solder. No other arrangements described above were 

 changed, and the method pursued was the same as before. 



The pressure of the hydrogen and oxygen was 25 mm. 

 The silver wire, whose resistance at 17° C. was about '73 ohm, 

 was heated for short intervals of time by currents of gradually 

 increasing magnitude until its resistance reached 1-91 ohms. 

 After this it gave way, being apparently fused at the point 

 of rupture. Meanwhile no trace whatever of chemical union 

 was indicated by the McLeod gauge. The only change of 

 pressure was a slight rise due to the heating of the gas. If 

 *0038 * is adopted as the value of the quantity a for this wire, 

 the resistance 1*91 indicates a temperature of about 470° 

 centigrade. 



It follows that no chemical action is produced by heating- 

 silver in hydrogen and oxygen far beyond the temperature to 

 which platinum must be raised in order to cause these gases 

 to unite. 



The chemical effect of the platinum is not, therefore, 

 merely produced by heating the gases in the neighbourhood 

 of the wire to a temperature at which their molecular 

 condition becomes unstable and passes into one of greater 

 stability. In other words, the temperature of the platinum 

 which sets up chemical action is far below the temperature at 

 which the gases would of themselves combine into water. 



The explanation must be that at the temperature in question 

 the platinum begins to exercise an action upon the hydrogen 

 and oxygen which paves the way for chemical union. 



Now it is well known that hot platinum discharges negative 



* This number was obtained by experiment upon a pure silver wire of 

 diameter -05 mm., also supplied by Messrs. Johnson & Matthey. It was 

 used for the same purpose as the larger one, but was much too fragile, 

 and broke after being heated to only 180° C. in hydrogen and oxygen at 

 35 mm. pressure. 



