hy the Immersion of unequally Heated Metals in Liquids. 7 



most perfect cleanliness is necessary in their preparation. The 

 more oxidable metals require to be cleaned at each experiment ; 

 and they should be used within a few minutes after being cleaned, 

 otherwise the deflections of the needles in the ensuing expei'i- 

 ment wiU be considerably decreased. Platinum plates only 

 require to be occasionally cleaned in ordinary experiments ; but 

 in all the comparative or more delicate experiments they should 

 be heated red-hot all over before each time of using. In com- 

 parative experiments it is of the highest importance that the 

 quantity of water in the cistern be equal in each case, also that 

 the current of steam be of uniform strength, and that the period 

 of time occupied in raising the temperature of the water from 

 the lowest to the highest point be accurately noted. 



18. The inside of the vessel should be occasionally examined 

 during experiments, and invariably when the water has risen in 

 temperature to nearly 200° F., to see if any air-bubbles remain 

 in the liquid, or are set free by the action of heat, or any gas 

 evolved by the action of the liquid upon the plates, or if any 

 action of the liquid has taken place upon the washers. With all 

 liquids, more or less of the atmospheric air dissolved in them is 

 set free against the upper plate at the higher temperatures ; but 

 this, with the metals I have used (aluminium, antimony, bismuth, 

 zinc, tin, iron, copper, silver, and platinum), only slightly affects the 

 amount of deflection at those temperatures, and disappears after a 

 few" examinations of the liquid, the air being expelled. I have re- 

 peatedly tried the effect of previously well boiling the liquid, and 

 cooling it in a closed glass flask quite filled with it ; but the de- 

 flections produced were but sUghtly different to those when the 

 liquid was not so prepared ; for delicate or comparative experi- 

 ments the liquid should always be pre-boiled. I have been 

 unable to examine those metals and hquids which evolve gas by 

 their miitual contact, although I have tried various contrivances 

 for the immediate removal of the gas ; or those in which the 

 metal in solution {i. e. with reducible metallic solutions) is re- 

 duced upon the plates by ordinary chemical action. 



19. Care must be taken to keep the outside of the apparatus 

 quite dry when in use, that the insulation of the plates from 

 each other may be as perfect as possible. 



Influence of various circumstances upon the Currents. 



20. I have proved that the ordinary thermo-electric property of 

 the metal has little or no influence upon the currents under con- 

 sideration, by interposing in the circuit with the galvanometer 

 the foregoing apparatus fitted with silver plates, and charged in 

 one instance with a j)re- boiled solution of carbonate of ])otash, 

 and in another instance with an acidulated solution of sulphate 



