It is hardly needful to point out too that on the earth's surface we live under nearly uniform conditions of gravity, temperature, and atmospheric pressure, so that if we are to extend our inferences to other parts of the universe where conditions may be widely different, we must be prepared to imitate those conditions on a small scale here. We must have intensely high and low temperatures we must vary the density of gases from approximate vacuum up wards we must subject liquids and solids to pressures or strains of almost unlimited amount.