"Curious relations, triads and octaves, were discovered to exist among the atomic weights of substances possessing similar physical characters and chemical properties. Chlorine, iodine, and bromine formed such a triad. Adding the atomic weights of chlorine and iodine, 35 and 125, the sum is 160. The mean or half, therefore, is 80, which is the atomic weight of bromine. These three substances are strictly analogous in their modes of combination, their salts are isomorphous, and with the increase in their atomic weight there is a uniform modification of their properties—chlorine is a green gas, bromine a very volatile red liquid, and iodine a slate-coloured solid, volatilised only by heat. Chlorine is the most active chemically, and can expel the others, bromine is next, and iodine is least active. Chloride of silver is easily soluble in ammonia, the bromide soluble with difficulty, and the iodide quite insoluble."