from Hot Platinum in Phosphorus Vapour. 411 



tap. the phosphorus vapour could be allowed to come in 

 contact with the hot wire, and the test made immediately. 

 In carrying out this experiment, it is important that the gas- 

 pressure on the two sides of the tap should be very low, say 

 about a hundredth of a millimetre, otherwise disturbances 

 might arise due to ionization by collision or — what it is still 

 more necessary to guard against — changes in the temperature 

 of the wire. 



The general arrangement of the apparatus will be made 

 clearer by reference to the accompanying figure. The tap A 

 separates the right-hand portion containing the phosphorus, 

 P. from the left-hand portion containing the hot wire, W. 

 B and B' are phosphorus-pentoxide drying-bulbs ; the main 



Electrometer 



pump is not shown. The left-hand side of the apparatus was 

 exhausted by means of the tap and bulb arrangement DEF. 

 The action of this is sufficiently obvious from the figure ; it 

 has been found very convenient where a pump is [required 

 for a purely temporary purpose. 



The following numbers refer to an experiment under these 

 conditions. A new platinum wire, cleaned by dipping into 

 nitric acid and heating in the Bunsen flame, on heating in a 

 good vacuum at a temperature of (i80° C, gave an initial leak 



- E 2 



