Separations, such as we just considered, turn upon obvious differences in density. Properties not obvious, yet highly useful, come into view year by year as observers grow more alert and keen, as new instruments are devised for their aid, as measurements become more refined, so that matter is constantly found to be vastly richer in properties than was formerly supposed. We have long known that carbon has forms which vary as widely as coal, graphite and the diamond. Many other elements are detected in a similar masquerade. Iron, for instance, takes three forms, _alpha_, _beta_, and _gamma_. _Alpha_ iron is soft, weak, ductile and strongly magnetic; _beta_ iron is hard, brittle and feebly magnetic; _gamma_ iron is also hard and feebly magnetic, yet ductile. Joule, the famous English experimenter, prepared an amalgam of iron with mercury; when he distilled away the mercury, the remaining iron took fire on exposure to the air, proving itself to be different from ordinary iron. Moissan has shown that similar effects follow when chromium, manganese, cobalt and nickel are released from amalgamation with mercury. At first steel was valued for its strength and elasticity; to-day we also inquire as to its conductivity for heat or electricity, its behavior in powerful magnetic fields, its capacity to absorb or reflect rays luminous or other. As art moves onward we enter upon new powers to change the properties of matter, compassing new intensities of heat and cold, each with new effects upon tenacity, elasticity, conductivity. So also with the extreme pressures, possible only with modern hydraulic apparatus, which prove marble to be plastic, and reduce wood to a density comparable with that of coal, explaining how anthracite has been consolidated from the vegetation of long ago.