184 Mr. J. Hopkinson on certain [Jan. 20, 



nexions were made through the reversing-key ; and, excepting the time ob- 

 servations for polarization, the readings were made twice at least on each 

 side of the zero-point. A single Daniell's element gave 105 divisions de- 

 flection each way, so that in the following the unit of electromotive force 

 j s i the electromotive force of Daniell's element. In all cases the 

 electrodes were platinum wires dipping into the fluid under examination. 

 In the experiments on polarization the circuit was readily closed for a 

 specified time by bringing the platinum wires into contact, and broken by 

 releasing them; the electromotive force could then be observed at any 

 instant after breaking the circuit. 



I. Strong sulphuric acid was poured into a test-tube, which dipped into 

 a porcelain crucible containing caustic potash. Thus the acid and alkali 

 were separated by the glass of the tube. Platinum electrodes dipped into the 

 two liquids. Electromotive force of 70 divisions was observed, the acid 

 being positive. The crucible was heated by a spirit-lamp till the potash 

 began to boil, the electromotive force increased to 153. The lamp was 

 removed and the crucible allowed to cool ; the electromotive force steadily 

 diminished to 78 in half an hour. The tube was then discharged and 

 insulated to observe the rate at which the charge developed. 



E.F. 

 15 seconds after insulation, 67 

 30 „ „ 69 



60 „ „ 69. 



II. The tube, crucible, and wire were thoroughly washed with cold water 

 and replaced in position, but with water in place of both the acid and the 

 alkali. The inside of the tube where the acid had been was now negative. 

 E. E.=16|-. Heating to boiling the deflection increased to 150 divisions, 

 but sank to 2 on cooling. 



III. The test-tube contained potash and dipped into water in the crucible. 

 When cold, E. E.=33, the potash being negative; when heated to boiling, 

 E. E.=36i 



IV. The tube contained strong sulphuric acid and dipped into water ; a 

 deflection of less than three divisions was observed. "When heated till the 

 water boiled, the reading was 35. After cooling the deflection decreased 

 to 5. 



V. The test-tube was removed and a small porcelain crucible introduced 

 in its place; sulphuric acid was poured into the outer crucible, potash into 

 the inner ; platinum electrodes dipped into the liquids. On heating till 

 the potash boiled, the electromotive force rose as high as 162. The de- 

 crease of the electromotive force as the liquids cooled was then observed. 



Time in minutes. E. F. 



155 boiling. 



32 94 still warm. 



91 88 quite cold. 



181 . , 88 



