304 Determination of Law of Reflexion of Gas Molecules. 



angles greater than about 80 degrees there appears to be 

 no reflexion, at least in the case o£ the two surfaces which 

 I have examined. 



The jet of molecules obtained in this way is interesting in 

 that both the temperature and pressure are vector quantities. 



Molecular collisions have been completely abolished and 

 the lateral pressure is zero. The same may be said of the 

 temperature I suppose, though we must specify what we 

 mean by temperature in such a case. 



An experiment was made to determine whether the jet 

 was capable of rendering its path conducting. The lower 

 constriction was made rather large, and a platinum wire 

 sealed in so as to lie across the path of the jet, which played 

 upon another electrode at the bottom of the bulb. The bulb 

 was non-conducting, and no visible discharge occurred when 

 the jet was started. It is possible, however, that experiments 

 more carefully carried out, on the electrical properties of a 

 one-dimensional gas, might yield interesting results. 



It is my intention to illuminate the jet with the light of 

 the 2536 mercury line, and determine the molecular velocity 

 by the displacement of the resonance radiation line. 



It may very well be that interesting results can be 

 obtained with an apparatus in which the reflecting surface 

 can be held at any desired temperature, for example at a 

 temperature a hundred degrees or. so higher than the tem- 

 perature at which the mercury vapour is liberated. Possibly 

 some information can be obtained in this way as to the 

 nature of the molecular agitation which we call heat. It is 

 obvious that the reflexion of the molecules results solely from 

 this agitation, since, if we remove the heat from the reflect- 

 ing body, reflexion no longer occurs and the molecules 

 remain fixed to the wall. It will also be interesting to work 

 at a temperature intermediate between that at which com- 

 plete reflexion occurs, and that at which complete condensa- 

 tion takes place. Experiments along these lines will be 

 undertaken in the autumn. 



I have not tried projecting the phenomenon, but see no 

 reason why it could not be done with a form of apparatus 

 similar to that shown in fig. 1 of the Plate. The success of 

 the experiment would depend largely upon whether the heat 

 rays which are absorbed by liquid air could be removed from 

 the beam of the lantern. 



