It has been stated that modern theory recognizes two classes of electricity, the _Static_ and the _Dynamic_. The difference is, however, solely noticeable in operation. Of the dynamic class there can be no more common and striking example than the now almost universal electric light. Yet, with a sufficient expenditure of chemicals and electrodes, and a sufficient number of cells, electric lighting, either arc or incandescent, can be as effectively accomplished as with the current evolved by a powerful dynamo. [Footnote: As an illustration of the day of beginnings, a few years ago the _thalus_, or lantern, the pride of the rural Congressman, on the dome of the Capitol at Washington was lighted by electricity, and an immense circular chamber beneath the dome was occupied by hundreds of cells of the ordinary form of battery. The lamps were of the incandescent variety, and what we now know as the filament was platinum wire. Vacuum bulb, filament, carbon, dynamo, were all unknown. But the current, and the heat of resistance, and every fact now in use in electric lighting, were there in operation.] The reader will understand that modern dynamic electricity owes its development to the principle of economy in production. Practical science most effectively awakens from its lethargy at the call of commerce. Nevertheless, from the earliest moment in which it became known that electricity was akin to heat--that an interruption of the easy passage of a current produced heat--the minds of men were busy with the question of how to turn the tremendous fact to everyday use. Progress was slow, and part of it was accidental. The great servant of modern mankind was first an untrained one. It was a marked advance when the gaslights in a theater could be all lighted at once by means of batteries and the spark of an induction coil. The bottom of Hell Gate, in New York harbor, was blown out by Gen. Newton by the same means, and would have been impossible otherwise. But these were only incidents and suggestions. The question was how to make this instantaneous spark _continuous_. There was pondering upon the fact that the only difference between heat and electricity is one of molecular arrangement. Heat is a molecular motion like that of electricity, without the symmetry and harmony of action electricity has. The vibrations of electricity are accomplished rapidly, and without loss. Those of heat are slow, and greatly radiated. _When a current of electricity reaches a place in the conductor where it cannot pass easily, and the orderly vibrations of its molecules are disturbed, they are thrown into the disorderly motion known as heat._ So, when the conductor is not so good; when a large wire is reduced suddenly to a small one; when a good conductor, such as copper, has a section of resisting conduction, such as carbon; heat and light are at once evolved at that point, and there is produced what we know as the electric light. However concealed by machinery and devices,