In order to demonstrate that radium throws off heat continually he took two glass vessels, one containing a thermometer and a tube of radium, the other containing a thermometer and no radium. Both vessels were closed with cotton, and it was presently seen that the thermometer in the vessel containing the radium registered constantly 5·4 degrees F. higher than the thermometer which was not so influenced. The most striking experiment presented by M. Curie in his London lecture was one devised by him to prove the existence of radium emanations, a kind of gaseous product (quite different from the rays) which this extraordinary metal seems to throw off constantly as it throws off heat and light. These emanations may be regarded as an invisible vapour of radium, like water vapour, only infinitely more subtle, which settles upon all objects that it approaches and confers upon them, for a time at least, the mysterious properties of radium itself. Thus the yellow powder sulphide of zinc bursts into a brilliant glow under the stimulus of radium emanations, and to make it clear that this effect is due to the emanations and not to the rays M. Curie constructed an apparatus in which a glass tube, R, containing a solution of radium is connected with two glass bulbs, A and B, containing sulphide of zinc. The experiment is begun by exhausting the air from the two bulbs A and B, by means of air-pump connections through the tube E. The air is not exhausted, however, from the tube R, over which the stop-cock F is closed, and within which the emanations have been allowed to accumulate. The room is now darkened, and it is seen that so long as the stop-cock F remains closed there is no glow in the bulbs A and B, but as soon as the stop-cock F is opened both bulbs shine brilliantly, so that the light is plainly visible at a distance of several hundred yards. Now, obviously, if this effect were due to the radium rays, it would be produced whether the stop-cock F were open or closed, since the radium rays pass freely through glass and need not follow the tube S in order to reach the bulbs A and B. It is therefore clear that the sudden light in the bulbs is due to the passage of _something_ out of the tube R, and through the tube S, that _something_ being kept back by the glass of the bulb R until the stop-cock F is opened. So we conclude that the emanations of radium _cannot_ pass through glass, and are a manifestation quite distinct from the rays of radium, which _can_ pass through but do not influence the sulphide of zinc. [Illustration]