Study of the Cohering Action of different Metals. 



169 



more difficult than with potassium, but the response is somewhat 

 similar to that of potassium. Though in general there is an increase 

 of resistance produced by electric radiation, there are occasional excep- 

 tions when a diminution of resistance is produced. With some trouble 

 the adjustment could be made so that the recovery is also automatic, 

 but it is not so energetic as in the case of potassium. 



Lithium. — Specimens of this metal not being available, I obtained a 

 deposit of it on iron electrodes by electrolysis of the fused chloride. 

 The action produced by electric radiation was sometimes an increase 

 and sometimes a diminution of resistance, the increase of resistance 

 being the more frequent. With some difficulty it was possible to 

 .adjust the sensitiveness so that the recovery was automatic, but it was 

 not energetic nor did this power persist for a long time. 



Metals of the Alkaline Earth. 



Pure metals of this group being not available, I had to rely on the 

 deposit obtained by electrolysis. Chloride of calcium was fused in a 

 crucible, and deposits were produced on iron cathodes, the anode being 

 a carbon rod. The deposit was not very even. One of the iron rods 

 with the deposit was tested by immersion under water, when hydrogen 

 was evolved. I did not succeed in getting deposits of either barium or 

 strontium, the temperature available not being sufficiently high. 



On making a coherer with calcium, and keeping it immersed in 

 kerosene, an action similar to that produced by sodium was observed. 

 The tendency of self-recovery was, however, very slight. 



Magnesium, Zinc, and Cadmium. 



In these metals and in the succeeding groups there is a pronounced 

 tendency towards a diminution of resistance by the action of electric 

 radiation. Magnesium being easily oxidisable, there is a thin coating 

 of oxide on the surface. When this is scraped, the metal makes a very 

 highly sensitive receiver. The adjustment is not difficult, the metal 

 allowing a considerable latitude of pressure and E.M.F. It has 

 already been stated that the metals which are slightly tarnished can 

 be more easily adjusted. 



Though there is in this metal a decided tendency towards a reduction 

 of contact resistance, yet it is possible by careful adjustment to obtain 

 an increase of resistance. Indeed it is sometimes possible to so adjust 

 matters that one flash of radiation produces a diminution of resistance, 

 and the very next flash an increase of resistance. Thus a series of 

 flashes may be made to produce alternate throws of the galvanometer 

 needle. The more stable adjustment, however, gives a diminution of 

 resistance, and receivers made with this metal could be made extremely 

 .sensitive. The tendency towards recovery is almost wanting. 



