1883.] 
Analyses of Books. 
235 
Faraday’s discovery of the specific indudlive capacity of different 
substances, but even determined its numerical value in several 
cases. As early as January, 1781, he anticipated the law of 
electric resistance, afterwards independently discovered by Ohm, 
and published by him in 1827. He experimented on the relative 
electric resistance of a series of acids and salts, and it is remark- 
able that the quantity of each substance used is “ very nearly 
the equivalent weight of that substance in the system adopted 
at present, in which the atomic weight of hydrogen is taken as 
unity.” The editor of the work before us thinks that, as these 
experiments were made in 1777, Cavendish must have obtained 
these numbers from determinations of his own, of the very ex- 
istence of which we have no other indication. 
In his memoir on the Torpedo, laid before the Royal Society 
in 1776, he treats of the different conductive powers, or rather 
of the resistances, of certain solids and liquids. He mentions 
that iron wire conducts about 400 million times better than rain 
or distilled water, and that sea-water, or a solution of 1 part of 
salt in 30 of water, condudts 100 times, and a saturated solution 
of sea-salt about 720 times better than rain-water. The editor 
remarks that this is equivalent to saying that iron wire condudts 
555,555 times better than a saturated solution of sea-salt. At a 
temperature of about n° C. ( = 52° F.) the ratio of the resistance, 
according to Matthiesen and Kohlrausch, would agree with that 
given by Cavendish. 
In the memoir aforesaid an account of the methods by which 
these numbers were obtained was promised. But it has lain 
buried till the publication of these papers. Cavendish was his 
own galvanometer, and compared the intensity of currents by 
passing them through his own body. How by so rude a 
method he obtained so close an approximation to the truth is a 
marvel. 
“The leading idea which distinguishes the electrical researches 
of Cavendish from those of his predecessors is the introduction 
of the phrase ‘ degree of electrification ’ with a clear scientific 
definition, which shows that it is precisely equivalent to what we 
now call potential.” 
Lord Stanhope’s “ Principles of Electricity,” a work very 
popular at one time, was evidently written in entire ignorance of 
what Cavendish had done. The latter devotes two pages to the 
refutation of the theory of electric atmospheres, which is the 
basis of Stanhope’s reasoning. 
Much has been said concerning the extent to which the 
researches of Coulomb have been anticipated by Cavendish. 
It is here pointed out that the method by which Coulomb made 
direct measurements of the electric force at different distances, 
and that by which he compared the density of the surface-charge 
on different parts of conductors, were entirely his own, and were 
not anticipated by Cavendish. But the very idea of the capacity 
