combines with 35.5 grains of chlorine may give the numerical value of the atomic weight of the element, if the compound contains one atom of each element; in that case the formula of the above compound would be zinc, and the atomic weight of zinc, 32.7; but if the formula is ZuCl3, the atomic weight of zinc would be 32.7 × 2; if ZuCl3, 32.7 × 3, and so on. The specific heat of metallic zinc enables this question to be solved. For it has been found, experimentally, to be about 0.095; and 6.2 ÷ 0.095 = 65.2, a close approximation to 32.7 × 2 = 65.4. The conclusion is therefore drawn that zinc chloride is composed of one atom of zinc in combination with two atoms of chlorine, that the atomic weight of zinc is 65.4, and that the molecular weight of zinc chloride is 65.4 + (35.5 × 2) = 136.4. Inasmuch as the relative weight of a molecule of hydrogen is 2 (that of an atom being 1), zinc chloride in the gaseous state should be 136.4 ÷ 2 = 68.2 times that of hydrogen, measured at the same temperature and pressure. This has been found, experimentally, to be the case. The methods of determining the vapor densities, or relative weights of vapors, are three in number; the first method, due to Dumas (1827), consists in vaporizing the substance in question in a bulb of glass or of porcelain, at a known temperature, closing the bulb while still hot, and weighing it after it is cold. Knowing the capacity of the bulb, the weight of hydrogen necessary to fill it at the desired temperature can be calculated, and the density of the vapor thus arrived at. A second method was devised by Gay-Lussac and perfected by A. W. Hofmann (1868); and a third, preferable for its simplicity and ease of execution, is due to Victor Meyer (1881). In 1858, as already remarked, Cannizzaro showed the connection between these known facts, and for the first time attention was called to the true atomic weights, which were, up to that time, confused with equivalents, or weights of elements required to replace one unit weight of hydrogen. These were generally regarded as atomic weights by Dalton and his contemporaries. Some exceptions had been observed to the law of Dulong and Petit, viz., beryllium, or glucinium, an element occurring in emeralds; boron, of which borax is a compound; silicon, the component of quartz and flint, and carbon. It was found by Weber that at high temperatures the specific heats of these elements are higher, and the atomic heats approximate to the number of 6.2; but this behavior is not peculiar to these elements, for it appears that the specific heat of all elements increases with rise of temperature.