In Nature of the 7th of March, Lord Kelvin reported the results of a series of experiments, which render it almost certain that Prof. J. Perry's hypothesis of the conductivity of rock increasing with the temperature is unsupported by evidence. He also stated that he had been led by later study to modify his original estimate of the age of the world, and to reduce it to about the same as estimated by Mr. Clarence King, viz. 24 million years. Thus geologists would find themselves restricted to a still shorter period for the evolution of the events of the world's history than they supposed.