_Ex._—This tube is filled with rarefied gases. Platinum wires convey the electric current through the tube, revealing curious striated sections of brilliant light, varying in shape and color, with the variety of gas and the degree of rarefaction.] Our sun is an immense reservoir of energy. It is difficult to conceive its size. It would require twelve hundred thousand of our globes to equal it in volume. More than one hundred such worlds as ours might be strung upon the line forming its diameter. The sun has been for ages throwing off its vibrations of heat and light. Thousands of years before fires were kindled on hearthstones this form of energy, according to the modern doctrine of the correlation of forces, was locked up in the tropical vegetation of the coal periods, and in the great deposits of coal preserved for future use. The same anticipatory benevolence which projects on its journey the friendly ray of the north star, forty-three years before the mariner’s eye can see it, provided fuel for man thousands of years before it was needed. This energy of the sunbeam reappears in the summer warmth of our dwellings in winter, in the expansion of steam, in the blow of the trip hammer, and throbs even in the pulsations of the human heart. The cells of all plants need the force of the sun’s rays to separate the carbon from the oxygen contained in the carbonic di-oxide absorbed by the rootlets and stomata of the leaves. Thus the great luminary builds the forests and clothes the earth with verdure. “All flesh is grass,” and therefore to the forces of the sun’s vibrations we must trace not a little of animal growth and strength. The sun gives out more heat than it would if six tons of coal were burnt on every square yard of its surface every hour. Sir John Herschel[4] declares that its light is equal to that of one hundred and forty-six calcium lights, each one formed of a ball of lime equal to the sun in bulk; yet even a small calcium light is so dazzling that the eye can not look steadily at it. The careless expression sometimes heard when the moon shines brightly, “It is as light as day,” is a striking hyperbole, for it would require eight hundred full moons to equal the brightness of daylight. [Illustration: ELECTRIC MOTION CONVERTED INTO SPARKS. _Ex._—A file is made part of the circuit, and as the wire conducting the electricity is rubbed along the file, the circuit is alternately formed and broken, and sparks follow each breaking of the circuit.]