680 Prof. 0. W. Richardson on th 



ie 



been quite unable to locate the determining cause of this 

 difference under conditions which were apparently identical. 



When the salt was heated to a higher temperature than 

 that at which observations were taken, it was always found 

 that the emission was greater than would otherwise have 

 been the case. This effect, which persists for some time, 

 has been observed, by Horton, to occur when sodium phosphate 

 is heated. 



No negative emission, comparable in amount with the 

 positive, was observed daring these experiments with sodium 

 sulphate, though it was frequently looked for when the 

 conditions seemed favourable. 



Experiments with a Platinum Tube. 



A general survey of the experiments on sodium sulphate 

 which have just been described, together with the rather 

 similar results obtained by previous observers working with 

 a similar type of apparatus on aluminium phosphate and 

 sodium phosphate, did not seem to point to any very definite 

 conclusion as to the nature of the action by which the pressure 

 of the gas gives rise to such great variations in the amount 

 of the positive emission. It seemed likely that more light 

 might be thrown on these phenomena if the measure- 

 ments were made under conditions differing as widely as 

 possible from those which had held previously. With this 

 object in view a new apparatus was constructed. The salt 

 (about one gram as a rule) was placed in the bottom of a 

 platinum tube which was closed at one end. The closed end 

 was tapered somewhat and the dimensions of the tube, 

 neglecting the taper, were: — length 25 cm., outside diameter 

 2 cm., and wall thickness '02 cm. In the early experiments 

 this tube was heated directly in a Heraeus platinum resistance 

 furnace, but it was found to collapse under the heat. It was 

 therefore placed, after trying several other materials, in a 

 mild steel tube which served as a crucible. The steel tube 

 w r as drilled out of 1-| inch mild rod, so that the platinum 

 tube went into it with a loose sliding fit. About 1^- inches 

 of the crucible was left solid at the bottom, and a hole about 

 1/2 an inch deep was drilled to receive the end of a platinum 

 platinum-rhodium thermo-couple. This was standardized 

 against a platinum resistance thermometer obtained from the 

 Cambridge Scientific Instrument Co., and known to be right. 

 The Heraeus tubular furnace was placed vertically, the 

 crucible and connexions being at the top, and the thermo- 

 couple coming out at the bottom. The ends were closed 

 with sheet asbestos, and the space between the porcelain 



