84. Dr. Beetz on the Electromotive Force of Gases, 

 in which 



The value found for R in both observations was assumed 

 to be constant during the whole series of observations, which 

 never occupied a period of more than two hours ; the same 

 applies to p. All those connexions which were not (like /■") 

 of equal value, or had been ascertained by calculation from 

 their resistance (as those contained in R), were effected by short 

 pieces of copper wire of the diameter of about a line. 



Before detailing the experiments in full, a few words more 

 must be said upon the preparation of the gas batteries. To 

 obtain the platinized platinum in its most effective state, the 

 plan proposed by M. PoggendorlF was followed ; the platiniza- 

 tion was effected in a dilute solution of chloride of platinum by 

 a current from two of Grove's cells, and thus a black coating 

 was produced. It will be subsequently seen, however, that in 

 the method of measuring adopted, a gray, imperfectly acting 

 precipitate cannot produce any great changes in the force of 

 the circuit, as the activity of the finely-divided layer is alto- 

 gether only a secondary one. If two platinum plates are 

 used to a gas battery, care must first be taken that they are 

 not capable of exciting any voltaic tension of themselves ; they 

 must be tested as to their being perfectly homogeneous ; and if 

 they are not, they must be made so. Serious error may be 

 fallen into in regard to this point, if the ordinary means are 

 used for connecting the plates with each other in the conduct- 

 ing fluid for any time. If, for instance, upon one of the plates 

 hydrogen in addition to the platinum is precipitated by a 

 somewhat strong current, as is usually the case, and this is 

 combined with a clean plate of })latinum to form the circuit, 

 hydrogen separates upon the latter plate; hence the current 

 will cease before the oxygen evolved at the former plate has 

 removed the hydrogen which it met with ; consequently the 

 plates appear to be homogeneous, but are far from consisting 

 of pure platinum. I always took care that the hydrogen was 

 first entirely dissipated, by causing all the plates about to be 

 used in a series of experiments to act as the anodes for a short 

 period. The oxygen and the chlorine by which they were 

 then coated are much more easily removed, either by connect- 

 ing all the plates, on the one hand, with another plate of pla- 

 tinum in a concentrated solution of platinum, so as to form a 

 circuit, or by boiling them for a long time with water, or what 

 is still better, by subjecting them to both these processes in 

 succession. The plates, which were about four inches in length 



