188 Dr. C. R. A. Wright and Mr. C. Thompson. [May 3, 



The spongy metals used were prepared as follows : — Silver sponge 

 (from acetate) by gently igniting in the air crystallised silver acetate ; 

 that from chloride by boiling well-washed silver chloride with sugar 

 and caustic soda until reduction was nearly complete. Spongy 

 palladium and platinum by gentle ignition of the ammonio-chlorides 

 of palladium and platinum respectively ; and spongy gold by gentle 

 ignition in the air for a long time (so as to burn off carbon) of cincho- 

 nine auro-chloride. The graphite was a very pure natural specimen 

 from Ceylon ; when used it was coarsely powdered, and spread over 

 the surface of porous earthenware like the spongy metals. As regards 

 the leaves and foils of silver, gold, and platinum, no discernible 

 differences could be distinguished between the values given by the 

 thinnest leaves and comparatively thick foils (up to O'l mm. in thick- 

 ness) in any of the three cases, saving tha£ the latter took a much 

 longer time before steady readings were obtained. Carbon (A) was 

 a piece of electric light rod ground down to a thin flat plate ; (B) 

 part of the carbon for a Leclanche cell similarly treated. 



It maybe noticed that some aeration plates composed of spongy 

 platinum with a top layer of platinum-black (precipitated from the 

 chloride by boiling with caustic soda and alcohol) gave figures pretty 

 close to those furnished by platinum sponge ; as also did other plates 

 consisting of porous earthenware painted over with platinochloride 

 of ammonium made into a paste with gum- water, ignited, and the 

 film of spongy platinum left on the surface burnished bright. 





Maximum. 



Minimum. 



Mean. 





1-473 

 1-455 

 1-491 



1-441 

 1-453 

 1-450 



1-457 

 1-454 

 1-463 



When dilute sulphuric acid was the fluid, however, the platinum - 

 black plates gave values upwards of a decivolt higher than sponge, 

 and the burnished pots about as much lower than sponge. 



A large number of observations were made with sets of aeration 

 plates and oxidisable metals in contact with caustic soda solution of 

 one strength subsequently changed for a different one, and so on, only 

 those readings being taken into account when steadiness was attained ; 

 thus the following figures were obtained where the plates were read 

 first inl-75Na 2 O,100H 2 O, then in 3-45Na 2 ,100H 2 O, then in 7-15Na 2 0, 

 100H 2 O, and then in the first again, and so on several times, so that 

 each plate was read several times in each strength of fluid. In all 

 cases the stronger the solution the higher the value, but the effect of 

 a given increment in solution- strength was very different with dif- 



