the subject of experiment: the amount of activity of a radio-active substance has not been increased or diminished by any process to which the substance has been subjected. Explosives are manufactured articles; explosiveness is a property of certain arrangements of certain quantities of certain elements: so far as experiments have gone, it has not been found possible to add the property of radio-activity to an inactive substance, or to remove the property of radio-activity from an active substance; the cessation of the radio-activity of an active substance is accompanied by the disappearance of the substance, and the production of inactive bodies altogether unlike the original active body. Radio-active substances are constantly giving off energy in the form of heat, sending forth _rays_ which have definite and remarkable properties, and producing gaseous _emanations_ which are very unstable, and change, some very rapidly, some less rapidly, into other substances, and emit _rays_ which are generally the same as the rays emitted by the parent substance. In briefly considering these three phenomena, I shall choose radium compounds as representative of the class of radio-active substances. Radium compounds spontaneously give off energy in the form of heat. A quantity of radium chloride which contains 1 gram of radium continuously gives out, per hour, a quantity of heat sufficient to raise the temperature of 1 gram of water through 100° C., or 100 grams of water through 1° C. The heat given out by 1 gram of radium during twenty-four hours would raise the temperature of 2400 grams of water through 1° C.; in one year the temperature of 876,000 grams of water would be raised through 1° C.; and in 1800 years, which is approximately the half-life period of radium, the temperature of 1,576,800 _kilograms_ of water would be raised through 1° C. These results may be expressed by saying that if 1 gram (about 15 grains) of radium were kept until half of it had changed into inactive substances, and if the heat spontaneously produced during the changes which occurred were caused to act on water, that quantity of heat would raise the temperature of about 15½ tons of water from its freezing- to its boiling-point.