I begin by directing the student to observe for himself the properties of bodies by which substances are distinguished. I place in his hands a bit of roll-brimstone. He first notices the color, the hardness, the brittleness, and the electrical excitability of this material. He next determines its density, its melting-point, its point of ignition, and, if practicable, its boiling-point. Then he treats the brimstone with various solvents, and finds that, while insoluble in water or alcohol, it dissolves readily in sulphide of carbon. Afterward he evaporates the solution thus made, and obtains definite crystals, whose forms he studies, and compares with the forms of the crystals of the same material which he also makes by fusion. Lastly, he observes the remarkable change which follows when fused brimstone is heated above its melting-point, and also the peculiar plastic condition which the material assumes when the thickened mass is poured into water. He will thus be led to see that the same material may assume different states, and gain a clear conception of the substance we call sulphur. After this I give the student pieces of two metals which externally resemble each other, like lead and tin, in order that, after making another series of observations and experiments, he may come to understand on what comparatively slight differences of properties the distinction between substances is frequently based. A comparison is next made of the properties of two closely-allied liquids, like methylic and ethylic alcohol; and by this time the student attains sufficient skill in experimenting to make a comparison between two aeriform substances, like oxygen gas and carbonic dioxide. After more or less of such preliminary work, we are prepared to take up the subject-matter of chemistry. In the broad fields of Nature what portion does this science cover? Natural phenomena may obviously be divided into two great classes: First, those changes which do not involve a transformation of substance; and, secondly, those changes whose very essence consists in the change of one or more substances into other substances having distinctive properties. The science of physics deals with the phenomena of the first class; the science of chemistry with those of the last. Any phenomenon of Nature which involves a change of substance is a chemical change, and in every chemical change one or more substances, called the factors, are converted into another substance or into other substances called the products. The first point to be made in teaching chemistry is, that a student should realize this statement, and a number of experiments should be shown in the lecture-room and repeated in the laboratory illustrating what is meant by a chemical change.