THE ELECTRICAL EXPERIMENTS OF HENRY CAVENDISH. 613 



second part of this Work," it must have been intended to be published as a 

 book, along with a reprint of that paper. It contains no dates, but as it 

 refers to experiments which we know were made in 1773, it must have been 

 written after that time, but I do not think later than 1775. 



It forms a scientifically arranged treatise on electricity. A manuscript entitled 

 " Thoughts concerning electricity" seems to form a kind of introduction to 

 this treatise, for it contains several important definitions and hypotheses which 

 are not afterwards repeated. 



Next comes the fundamental experiment, in which it is proved that a 

 conducting sphere insulated within a hollow conducting sphere does not become 

 charged when the hollow sphere is charged and the inner sphere is made to 

 communicate with it. 



Cavendish proves that if this is the case, the law of force must be that 

 of the inverse square, and also that if the index instead of being 2 had 

 been 2 + ^g-, his method would have detected the charge on the inner sphere. 



The experiment has been repeated this summer by Mr MacAlister of St 

 John's College with a delicate quadrant electrometer capable of detecting a 

 charge many thousand times smaller than Cavendish could detect by his straw 

 electrometer, so that we may now assert that the index cannot exceed or 

 fall short of 2 by the millionth of a unit. 



The second experiment is a repetition of this, using one parallelepiped within 

 another instead of the two spheres. 



He then describes his apparatus for comparing the charges of different 

 bodies, or, as we should say, their capacities. 



He first shews (Exp. 3) that the charge, communicated to a body con- 

 nected to another body at a great distance by a fine wire, does not depend 

 on the form of the wire, or on the point where it touches the body. 



Exp. 4 is on the capacities of bodies of the same shape and size but of 

 different substances. 



Exp. 5 compares the capacity of two circles with that of another of twice 

 the diameter. 



Exp. 6 compares the capacity of two short wires with that of a long one. 



Exp. 7 compares the capacities of bodies of different forms, the most im- 

 portant of which are a disk and a sphere. 



Exp. 8 compares the charge of the middle of three parallel plates with 

 that of the outer plates 



