158 DESCRIPTION OF THE FIGURES, 



Fig. 7. Represents the apparatus for taking the galvanic 

 spark in gases : it consists of a graduated glass tube, into 

 which two wires are introduced, the one which enters at the 

 side being moveable, and capable of being approached to the 

 other ; according to circumstances, they may be tipped with 

 pieces of charcoal, or the wires may be bare. 



Fig. 8. Is a variation in the form, which may be em- 

 ployed over mercury. These instruments were invented by 

 Sir H. Davy. 



Figs. 9, 10, and 11. Consist of a representation of Mr. 

 Pepys's galvanometer ; Fig. 9, is the apparatus in its complete 

 state, except that the lid is elevated a little above the mouth 

 of the glass cylinder, in order to render the description more 

 perspicuous. The lid is composed of two circular plates of 

 brass, attached to a cork, that fits into the cylinder ; a thin 

 slip of silver is fixed to the lid, the end of which hangs down 

 in the body of the cylinder and has a pair of gold leaves 

 attached to it, and the whole is so contrived as to be capable 

 of being moved nearer to, or farther from, the pieces of zinc, 

 which stand up from the bottom of the cylinder. In Fig. 10, 

 the under sicje of the co^er is seen, and also a section of it, 

 with the slip of silver adapted to it. The pieces of zinc are 

 BO contrived that the parts of them which project upwards 

 from the bottom of the cylinder may be fixed at different 

 distances from each other, by means of a slide and screw, as 

 is represented in Fig. 11. 



Fig. 12. Represents the apparatus employed by M. De 

 Luc to illustrate his theory of the galvanic pile: it con- 

 sists of two piles connected by a metallic rod at the bottom ; 

 between the upper ends is interposed the interrupted wires 

 terminating in water, and to each extremity one of Bennet's 

 electrometers is applied. 



Fig. 13. Represents the apparatus which Sir H. Davy 

 employed for the decomposition of salts, and the transfer of 

 their constituents. 



Fig. 14. Represents the luminous arch of light produced by 

 passing the galvanic influence between two portions of char- 

 coal. 6 



