390 Dr. R. A. Lehfeldt on Electromoti 



ve 



flocculent black deposit, the nature of which was not 

 determined with certainty, but 'was probably metallic indium. 

 The deposit appeared to make no difference to the electro- 

 motive force, and after repeated cleaning of the rods gradually 

 ceased. 



The zinc sulphate contained no iron, or other appreciable 

 impurity ; solutions were made up from it, as obtained, and 

 merely filtered into stock bottles. Several strong solutions 

 were weighed out direct : those below normal strength were 

 made by diluting a strong solution with pipette and measure- 

 flask. 



The zinc chloride, in the form of fused sticks, contained 

 no appreciable impurity except zinc hydroxide : a strong 

 solution of the chloride was found to dissolve an appreciable 

 amount of the hydroxide, which is precipitated on dilution. 

 To obtain a neutral solution the sticks were dissolved in 

 water and hydrochloric acid added till the liquid just began 

 to turn litmus red : the density was taken, and the concen- 

 tration calculated from the tables in Kohlrausch and Holborn's 

 Leitvermogen der Elektrolyte : weaker solutions could be pre- 

 pared by dilution, without further inconvenience. 



The mercurous sulphate (Harrington's) was that used for 

 Clark cells ; it remained perfectly white in use. The mer- 

 curous chloride at first contained traces of mercuric chloride, 

 but on washing proved quite satisfactory. 



Cells. — The cells used are here referred to for brevity as 

 " mercury " and " concentration " cells. The former are of 

 the types, 



Zn:ZnCl 2 : Hg 2 Cl 2 : Hg, 

 Zn : ZnS0 4 : Hg 2 S0 4 : Hg, 

 the latter 



Zn : ZnCl 2 (dilute) : ZnCl 2 (cone.) : Zn, 

 Zn : ZnS0 4 (dilute) : ZnS0 4 (cone.) : Zn. 



The " mercury " cells were of two kinds, " rod " and 

 "amalgam." The rod cells were made up in short wide 

 test-tubes, closed by corks with two borings : one of these 

 held in place the glass tube through the end of which a 

 platinum wire was sealed for communication with the 

 mercury : when the mercury had been poured in and the 

 platinum fixed in place, the depolarisator and the solution 

 were added through the other boring, and lastly the zinc rod 

 fixed in it : the platinum wire consequently remained . clean 

 and dry. The amalgam cells were made up in H form, with 

 similar precautions. 



