PHYSICS IN 1913 623 



the work described in the next paragraph it seems likely that 

 the whole effect is due to traces of gas. 



At the suggestion of Freydenhagen a student of his, Kiistner, 

 has studied the photoelectric effect in zinc, one of the metals 

 hitherto supposed to be particularly active in this respect, which 

 had been carefully purified and scraped by means of a magnetic- 

 ally actuated blade while actually in a vacuum. Special means 

 were resorted to in order to obtain a very high vacuum, and to 

 rid the zinc of the last traces of the gas ; Freydenhagen had 

 come to the conclusion that under these circumstances the photo- 

 electric effect would cease. It was found experimentally that by 

 prolonged scraping in vacuo and exhaustion the effect could be 

 reduced until it was not detectable ; further, that the various 

 types of photoelectric fatigue and abnormal initial effects were 

 easily explained on the assumption that they were due to the 

 occlusion of residual gases, and could be imitated at will. The 

 results are striking enough, and present remarkable similarities 

 throughout with those of Pring on the different effect. The 

 work is being extended in both directions, and, while it is as yet 

 too early to dogmatise, it certainly seems probable that the 

 emission of electrons due to the action of both heat and illumina- 

 tion by ultraviolet light is bound up with and dependent on the 

 presence of occluded gases. The many irregularities observed 

 by workers in these fields confirm this belief. 



The foregoing does not pretend to be a complete record of 

 all work of any importance done during the past year, but rather 

 an account of certain pieces of work performed, and theories put 

 forward, during that period, selected because they seem likely to 

 prove of far-reaching importance and to modify and extend our 

 existing ideas. Thus there can be no doubt that the methods 

 of investigating the X-rays opened up by Laue and the Braggs, 

 father and son, have already led to results of fundamental import- 

 ance, and are likely to lead to many more. Much careful work 

 done in elaboration of older lines of research has been passed 

 over without mention, not because the author is possessed of 

 that passion for the new which to-day seems in so many cases to 

 express itself in a desire rather to tear down than to build up, 

 but because considerations of space have prohibited an adequate 

 treatment of more than a relatively few selected themes. It is 

 to be hoped that the work here described will all of it prove of 

 permanent value to the progress of physics. 



