Electricity and Galvanism. 253 



formed by the union of the negative electricity of the body, which is 

 held near the conductor, with the free positve electricity of the 

 latter. 



In addition to the electrical machine itself, a pair of directors, or 

 rods of brass, furnished with balls of brass and glass handles, toge- 

 ther with a few yards of common copper bell-wire, or brass chain, 

 will be required to connect the patient with the machine, or to 

 convey the discharge of a jar through his body. The jar itself need 

 not have more than a square foot of coated surface, and indeed one 

 much smaller is often sufficient. 



There is one piece of apparatus which is very essential, being in 

 almost constant requisition, — I mean the well-known chair with glass 

 legs, on which a patient may sit and be completely isolated from all 

 electrical communication with the earth. This is an expensive, 

 bulky, and fragile contrivance, and hence is the most inconvenient of 

 all the electrical appliances. I advise you, however, not to trouble 

 yourselves with this very clumsy chair, which you will generally find 

 at the instrument makers ; as any ordinary chair can be at once 

 rendered most effectual in insulating any person, by merely placing 

 each of its four legs in a thick cup of glass. These may be procu- 

 red at any of the glass shops by merely asking for four thick, round, 

 glass salt-cellars in the rough state in which they are sent from the 

 glass-house before being engraved or cut. Thus, at the expense of a 

 couple of shillings, any comfortable chair may be converted into an 

 excellent insulating support. 



Galvanic electricity, or that excited by chemical action, is some- 

 times called in requisition. There are, however, many serious in- 

 conveniences attending its employment ; and not the least of these 

 is the bulky and unmanageable form of apparatus required for its 

 excitation in a state of even moderate tension. On this account 

 this form of electricity is now seldom employed, and in my own 

 practice I confess I never use it ; for the electricity of dynamic 

 induction is so much easier excited, and, being the same in essence, 

 has always been, so far as my own experience has extended, sub- 

 stituted for it. Whenever you wish to employ this form of elec- 

 tricity, you will find no apparatus more convenient for its excitation 



