Presents. 1896.] maintained at a temperature of 251° (which is 75° below the melting point of lead) for thirty-one days. At the end of this period the solid lead was cut into sections, and the amount of gold which had diffused into each of them was determined in the usual way. Other experiments follow, in which the lead was maintained at 200°, and at various lower temperatures down to that of the laboratory. The following are the results :-Diffusivity of gold in fluid lead at 550°h . 319 99 solid 2 5 1°.... 0-03 99 99