But radium raises many other questions affecting the nature of light and heat, such as how such great heat and light can be condensed into so small a compass and evolved with such wonderful power. Prospecting for valuable metals by electricity has been recently introduced in Wales with remarkable success. In the Cwnystwyth mines in Cornwall new and valuable deposits of ore and blende were located. These mines have been worked for over fifteen hundred years, and much of this time the search for additional lodes has been going on; yet this electrical device accomplished in a few hours what fifteen centuries of search failed to discover. By the electric method of prospecting a current of high potential--30,000 volts or more--is employed to energize a piece of ground supposed to contain mineral. The current is taken from the terminals of the generating coil to metal rods or electrodes which are pushed an inch or two into the earth. From these distributers the lines of force spread out in both vertical and horizontal planes and may be made to extend over an area of several miles. Their presence is detected by means of a delicate telephonic receiver connected with a second pair of metallic rods, which are stuck into the earth in any desired position. When the receiver is silent or gives only faint sounds, it indicates that the lines of force are deflected from their normal course. This reveals the presence of metallic masses in the earth, and, by moving the electrodes about, the position and area of an ore deposit may be determined with considerable accuracy. In new and unknown ground the operator would place his distributing electrodes about 200 yards apart, and remove his receiving instrument to a distance of half a mile or more. If his general knowledge of geological formation led him to believe that the metallic veins, if there were any, would run generally north and south, he would place his electrodes east and west, and if he found the electrical distribution normal he might conclude that the ground contained no mineral. This remarkable electrical system for detecting the mineral deposits beneath the surface of the earth will doubtless be tested and perfected until man will be able without digging into the earth to ascertain the mineral deposits at any point beneath its surface. It may not always determine if they may be profitably worked, but in many mining regions it will be a blessing, and save much unprofitable work and sad disappointment.