4. “Carboniferous period.” Coal age. By careful study it has been found that in the progress of the earth’s development a number of great ages have existed—each distinguished from the others by some marked change. That of coal plants is placed by geologists as the fourth age, counting upward from the lowest formation. It was remarkable for the alternate low elevation of the land above the sea level, and its submergences; and also for the luxuriant growth of vegetation, which, under the great pressure and heat to which it was subjected while the surface was submerged, was changed into coal. 5. “Spectroscope.” The name given to the apparatus used for the study of the spectrum. “When a ray of sunlight admitted through an aperture in a dark room is concentrated upon a prism of rock salt”—or glass—“by means of a lens of the same material, and then after emerging from the prism is received on a screen, it will be found to present a band of colors, in the following order: red, orange, yellow, green, blue, indigo, and violet.”—_Ganot_. This band is called the spectrum. That there are other but invisible lines than those mentioned in the spectrum, is proven by the use of the thermopile, oftener called thermomultiplier, mentioned in the article on “Home Studies in Chemistry.” This is a complicated instrument used for detecting minute differences in the degrees of heat; its description without an accompanying illustration would be of no benefit to any one. So delicate is it that the heat of the hand held at a distance of three feet is sufficient to deflect the needle. The spectroscope is composed of three telescopes, mounted on a common foot, whose axes converge toward a glass prism. One of the telescopes is movable, and can be adjusted so as to give the observer the clearest view of the spectrum. The ray of light is admitted through the telescope and falls upon the prism, which decomposes it, and the spectrum is formed on the opposite side of the prism. In the telescope which the observer uses a powerful magnifying glass is placed. The third telescope is used for measuring the relative distances between the lines. HOME STUDIES IN CHEMISTRY. 1. “James Dwight Dana.” (1813-⸺.) An American geologist and mineralogist; the author of several works on these and other sciences. He went out as mineralogist and geologist appointed by the United States Government with the exploring expedition sent to the Southern and Pacific Oceans in 1838, and returned in 1842. For the next fourteen years he was busily engaged in preparing for publication the reports of this exploration. These were published by the government and formed valuable records. For many years he was one of the editors of the _American Journal of Science and Arts_. He has been elected to membership in several learned European societies and royal academies.