50 MR. DANIELL ON VOLTAIC COMBINATIONS. 



diameter, I tied a piece of membrane : the tube was then filled with dilute acid and 

 plunged into the solution of copper contained in the brass hemisphere, against the 

 bottom of which it rested. The ball was supported within half an inch of the dia- 

 phragm. Upon closing the circuit by means of the magnetic galvanometer, the 

 needle indicated 40°. At the expiration of five minutes the solution was drawn off 

 and the hemisphere examined: there was found a beautiful well-defined circle of 

 pink copper, two inches diameter, surrounded by a halo of darker colour, evidently 

 of fresh precipitated copper of less thickness, but not extending over more than a 

 fourth part of the surface. From this experiment it was evident that the force had 

 diverged from the aperture of the glass tube, as from a centre, after it had entered the 

 solution ; the circle of pink copper being of a diameter half an inch greater than the 

 aperture, and the fainter halo extending some inches around. 



I now moved the tube into such a position, that, the zinc ball, remaining where it 

 was in the tube, might be just below the level of the solution in the hemisphere ; and 

 now, notwithstanding the greater distance at which it was placed from the conducting 

 surface, the needle rose to 45°, and the precipitated copper made its appearance over 

 the whole hemisphere. 



While the zinc ball was in its last position, the glass tube itself was pressed down 

 till it again rested upon the bottom of the hemisphere ; all lateral diffusion was thus 

 cut off for a distance of 4J inches, through which the force was propagated, and it 

 could only spread after it emerged from the tube ; the galvanometer indicated 30°. 

 When the glass screen, as it may be called, was again drawn up, the needle returned 

 to 45°. 



Similar experiments were often repeated with the substitution of large flat plates 

 for the hemisphere. When the zinc ball was thus confined in a tube with a diaphragm, 

 and placed within an inch of the plate, the precipitation always commenced with a 

 circle a little larger than the aperture, and gradually extended itself, so that after 

 some hours action it formed a circle of four or five inches diameter, and sometimes 

 turned the edge, and made its appearance on the under surface. 



Being desirous of ascertaining to what amount the under surface of a plate thus 

 immersed in the electrolyte would affect the action, I covered the upper surface of 

 the oval copper plate with lac varnish, and replaced it in its pan ; I then placed the 

 zinc ball in the tube within half an inch of the diaphragm, and plunged it just below 

 the surface of the solution ; upon completing the circuit the needle indicated 50°. In 

 ten minutes time I examined the plate, and found the under side covered with beau- 

 tiful pink copper, with the exception of an irregular oval space, whose diameters were 

 about 4^ inches by 4. The precipitation had evidently begun upon the edge, where 

 it was thickest. 



The plate was returned to its position, and the galvanometer again indicated 50°. 

 When a double ball was substituted for the single it rose to 55°. It was again ex- 

 amined after an hour's action, and on the under side presented the appearance of a 



