ELECTRICITY IN MOTION. 31 



gunpowder. The peculiar sensation which the electric fluid occa- 

 sions in the human frame, appears in general to be derived from 

 the spasmodic contractions of the muscles through which it passes ; 

 although in some cases it produces pain of a different kind ; thus, 

 the spark of a conductor occasions a disagreeable sensation in the 

 skin, and when an excoriated surface is placed in the galvanic cur- 

 rent, a sense of smarting, mixed with burning, is experienced. 

 Sometimes the effect of a shock is felt most powerfully at the joints, 

 on account of the difficulty which the fluid finds in passing the 

 articulating surfaces which form the cavity of the joints. The 

 sudden death of an animal, in consequence of a violent shock, is 

 probably owing to the immediate exhaustion of the whole energy 

 of the nervous system. It is remarkable that a very minute tre- 

 mor, communicated to the most elastic parts of the body, in par- 

 ticular to the chest, produces an agitation of the nerves, which is 

 not wholly unlike the effect of a weak electricity. 



The principal modes in which the electric equilibrium is prima- 

 rily destroyed are, simple contact, friction, a change of the form 

 of aggregation, and chemical combinations and decompositions. 

 The electricity produced by the simple contact of any t\vo sub- 

 stances is extremely weak, and can only be detected by very de- 

 licate experiments : in general it appears that the substance, which 

 conducts the more readily, acquires a slight degree of negativ< j 

 electricity, while the other substance is positively electrified in an 

 equal degree. The same disposition of the fluid is also usually 

 produced by friction, the one substance always losing as much a 

 the other gains ; and commonly, although not always, the wor>r 

 conductor becomes positive. At the instant in which the friction 

 is applied, the capacities or attractions of the bodies for electricity- 

 appear to be altered, and a greater or less quantity is required 

 for saturating them ; and upon the cessation of the temporary 

 change, this redundancy or deficiency is rendered sensible. When 

 two substances of the same kind are rubbed together, the smaller 

 or the rougher becomes negatively electrified; perhaps because 

 the smaller surface is more heated, in consequence of its under* 

 going more friction than an equal portion of the larger, and hence 

 becomes a better conductor ; and because the rougher in itself is 

 a better conductor than the smoother, and may possibly have its 

 conducting powers increased by the greater agitation of its parts 

 which the friction produces. The back of a live cat becomes 



