between the two. The phenomena which we have described are, therefore, due to this electrical separation, and inasmuch as the two electricities have a great affinity for one another, it requires the expenditure of energy to produce this separation, just as truly as it does to separate a stone from the earth.
84. Now, it is worthy of note that electrical separation is only produced when heterogeneous bodies are rubbed together. Thus, if flannel be rubbed upon glass, we have electricity; but if flannel be rubbed upon glass covered with flannel, we have none. In like manner, if silk be rubbed upon sealing-wax covered with silk, or, in fine, if two portions of the same substance be rubbed together, we have no electricity.
On the other hand, a very slight difference of texture is sometimes sufficient to produce electrical separation. Thus, if two pieces of the same silk ribbon be rubbed together lengthwise, we have no electricity; but if they be rubbed across each other, the one is positively, and the other negatively, electrified.
In fact, this element of heterogeneity is an all important one in electrical development, and this leads us to conjecture that electrical attraction may probably be regarded as peculiarly allied to that force which we call chemical affinity. At any rate, electricity and chemical affinity are only manifested between bodies that are, in some respects, dissimilar.
85. The following is a list of bodies arranged according
