83. These facts have given rise to a theory of electricity, or at least to a method of regarding it, which, if not absolutely correct, seems yet to unite together the various phenomena. According to this hypothesis, a neutral, unexcited body is supposed to contain a store of the two electricities combined together, so that whenever such a body is excited, a separation is produced 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 to the electricity which they develop when rubbed together, each substance being positively electrified when rubbed with any substance beneath it in the list. 1. Cat’s skin. 2. Flannel. 3. Ivory. 4. Glass. 5. Silk. 6. Wood. 7. Shellac. 8. Resin. 9. Metals. 10. Sulphur. 11. Caoutchouc. 12. Gutta-percha. 13. Gun-cotton. Thus, if resin be rubbed with cat’s skin, or with flannel, the cat’s skin or flannel will be positively, and the resin negatively, electrified; while if glass be rubbed with silk, the glass will be positively, and the silk negatively, electrified, and so on. 86. It is not our purpose here to describe at length the _electrical machine_, but we may state that it consists of two parts, one for generating electricity by means of the friction of a rubber against glass, and another consisting of a system of brass tubes, of considerable surface, supported on glass stems, for collecting and retaining the electricity so produced. This latter part of the machine is called its _prime conductor_. _Electric Induction._ [Illustration: Fig. 6.]