Iodine is a dark purple solid, crystallized in scales or elongated octohedral plates. It slowly evaporates at ordinary temperatures, and at that of 350° Fahr. it is volatilized into a beautiful violet coloured gas which changes starch into a bright blue, and for that reason a little starch will detect the millionth of a grain of iodine in composition. Iodine is slightly soluble in water, has a hot acrid taste, and although used in medicine it is poisonous when taken in large doses. Its bleaching properties are inferior to those of its congeners, but its chemical combinations are the same. With hydrogen it forms a highly explosive compound, which detonates with the slightest pressure. These three simple substances are analogous in almost every respect. They all possess a bleaching property, many of their compounds are exceedingly explosive, combustible substances do not burn in their gases, while their gases set fire spontaneously to substances generally reckoned incombustible. Hence, though not combustible, they support combustion, but in a very different manner from oxygen. Chlorine and the gases of bromine and iodine diluted with common air, do not transmit blue and violet light; that is to say, the spectrum of a sunbeam transmitted through them is deprived of its most refrangible coloured rays, and that which remains is crossed by more than a hundred equidistant dark lines; their spectral properties however will be given hereafter. They resemble oxygen in one respect—that when a current of electricity is passed continuously through a glass tube filled with any of these three gases, much attenuated, they slowly combine with the platinum wire of the negative pole of the battery inserted in the tube. The electricity by degrees passes in diminished quantity, and at last ceases altogether, showing that matter, however attenuated, is requisite to conduct it. According to the experiments of M. Dumas, the volatility of a compound is in the inverse ratio of the condensation of the substances composing it, and simple bodies come under the same law. For example, chlorine is more volatile than bromine, and bromine is more volatile than iodine; hence according to that law, chlorine is the least dense of the three, bromine is intermediate, and iodine is the most dense, which is actually the case: for chlorine is a gas, bromine a liquid, and iodine a solid at ordinary temperatures, which proves that there is a sequence in the intensity of the cohesive forces in this triad. SECTION II. ON FORCE, AND THE RELATIONS BETWEEN FORCE AND MATTER.