CH. xui CHEMICAL CHANGE BY ELECTRIC CURRENT 199 



manently joined together by a suitable connection such as 

 copper wire. Notice that in whatever direction the astatic 

 pair points there is no disposition for it to change this 

 direction ; whereas the single magnetic needle will only rest 

 in one direction, called the magnetic meridian. Of course, if 

 one of the needles is magnetically stronger than the other 

 they will not exactly counterbalance one another. 



EXPT. 193. Arrange the astatic pair and a coil of wire as 

 in Fig. 105. Notice that a very slight current, such as that 



FIG. 104. An Astatic Pair of Mag- 

 netic Needles. 



FIG. 105. An Astatic 

 Galvanoscope. 



obtained by dipping a brass pin and a steel needle into dilute 

 sulphuric acid, is enough to cause a deflection of the needle. 



The Passage of the Electric Current through Liquids. 

 EXPT. 194. Fit up a Buiisen's cell for the generation of an 

 electric current. Attach pieces of platinum foil, by means of 

 suitable binding screws, to the ends of two copper wires. 

 Attach one of these wires to one pole of the battery. Connect 

 the other pole to one of the binding screws of a simple galva- 

 noscope, and to the other screw of the galvanoscope attach 

 the remaining wire with the platinum plate on the end (see 

 Fig. 107). Dip the platinum plates 1st, into some mercury, 

 and notice there is a great deflection of the needle of the 

 galvanoscope and no alteration of the mercury ; 2nd, into 

 some turpentine, and notice there is no deflection of the 

 needle ; 3rd, into some acidulated water, and notice there is a 

 smaller deflection than in the first case, and at the same time 

 there are bubbles of gas given off" from both platinum plates. 



1st Case. Passage of the Current through Mercury. The 

 student will shortly learn (Chap. XIV} that mercury is one of 



