energy of position which we can draw upon with more facility than if it were a head of water, for, although we can draw upon the energy of a head of water whenever we choose, yet we cannot carry it about with us from place to place as we can with coal. We thus perceive that it is not the coal, by itself, that forms the source of energy, but this is due to the fact that we have coal, or carbon, in one place, and oxygen in another, while we have also the means of causing them to unite with each other whenever we wish. If there were no oxygen in the air, coal by itself would be of no value.

Electricity: its Properties.

79. Our readers have now been told about the force of cohesion that exists between molecules of the same body, and also about that of chemical affinity existing between atoms of different bodies. Now, heterogeneity is an essential element of this latter force-there must 'be a difference of some kind before it can exhibit itselfand under these circumstances its exhibitions are frequently characterized by very extraordinary and interesting phenomena.

We allude to that peculiar exhibition arising out of the forces of heterogenous bodies which we call electricity, and, before proceeding further, it may not be out of place to give a short sketch of the mode of action of this very mysterious, but most interesting, agent.

80. The science of electricity is of very ancient origin;

but its beginning was very small.

For a couple of thou

sand years it made little or no progress, and then, during the course of little more than a century, developed into the giant which it now is. The ancient Greeks were aware that amber, when rubbed with silk, had the property of attracting light bodies; and Dr. Gilbert, about three hundred years ago, showed that many other things, such as sulphur, sealing-wax, and glass, have the same property as amber.

In the progress of the science it came to be known that certain substances are able to carry away the peculiar influence produced, while others are unable to do so; the former are called conductors, and the latter non-conductors, or insulators, of electricity. To make the distinction apparent, let us take a metal rod, having a glass stem attached to it, and rub the glass stem with a piece of silk, care being taken that both silk and glass are warm and dry. We shall find that the glass has now acquired the property of attracting little bits of paper, or elder pith; but only where it has been rubbed, for the peculiar influence acquired by the glass has not been able to spread itself over the surface.

If, however, we take hold of the glass stem, and rub the metal rod, we may, perhaps, produce the same property in the metal, but it will spread over the whole, not confining itself to the part rubbed. Thus we perceive that metal is a conductor, while glass is an insulator, or non-conductor, of electricity.

81. We would next observe that this influence is of two kinds. To prove this, let us perform the following experiment. Let us suspend

Fig. 5.

a small pith ball by a very slender silk thread, as in Fig. 5. Next, let us rub a stick of warm, dry glass with a piece of warm silk, and with this excited stick touch the pith ball. The pith ball, after being touched, will be repelled by the excited glass. Let us next excite, in a similar man

ner, a stick of dry sealing-wax with a piece of warm, dry flannel, and on approaching this stick to the pith ball it will attract it, although the ball, in its present state, is repelled by the excited glass.

Thus a pith ball, touched by excited glass, is repelled by excited glass, but attracted by excited sealing-wax.

In like manner, it might be shown that a pith ball, touched by excited sealing-wax, will be afterwards repelled by excited sealing-wax, but attracted by excited glass.

Now, what the excited glass did to the pith ball was to communicate to it part of its own influence, after which the ball was repelled by the glass; or, in other words, bodies charged with similar electricities repel one another.

Again, since the pith ball, when charged with the electricity from glass, was attracted to the electrified sealingwax, we conclude that bodies charged with unlike electricities attract one another. The electricity from glass is sometimes called vitreous, and that from sealing-wax resinous, electricity, but more frequently the former is known as positive, and the latter as negative, electricity— it being understood that these words do not imply the possession of a positive nature by the one influence, or of a negative nature by the other, but are merely terms employed to express the apparent antagonism which exists between the two kinds of electricity.

82. The next point worthy of notice is that whenever one electricity is produced, just as much is produced of an opposite description. Thus, in the case of glass excited by silk, we have positive electricity developed upon the glass, while we have also negative electricity developed upon the silk to precisely the same extent. And, again, when sealing-wax is rubbed with flannel, we have negative electricity developed upon the sealing-wax, and just as much positive upon the flannel.

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