PHOSPHORESCENCE AND FLUORESCENCE.--Although the word phosphorescence has been used in a very loose way to indicate all kinds of luminescence, and particularly that of phosphorus or of luminous animals, to the physicist it has a very definite meaning, namely, the absorption of radiant energy by substances which afterwards give this off as light. Phosphorescence does not strictly apply to the light of white phosphorus. If the radiant energy is light (visible or ultra-violet) we speak of _photoluminescence_, if cathode rays we have _cathodoluminescence_, if anode rays, _anodoluminescence_, and if X-rays (Röntgen rays) we have _radioluminescence_. Inasmuch as the α, β, and γ rays of radium correspond to the anode, cathode, and X-rays, respectively, radium radiation also produces luminescence in many kinds of material. If the material gives off the light only during the time it is radiated we speak of fluorescence; if the light persists we speak of phosphorescence. The distinction is perhaps a purely arbitrary one, as there are a great many substances which give off light for only a fraction of a second (1/5000 sec. in some cases) after being illuminated (_photoluminescence_). Some substances also, which fluoresce at ordinary temperatures, will phosphoresce at low temperatures. Phosphorescence is exhibited chiefly by solids, fluorescence also by liquids and vapors. Special means must be used to observe a phosphorescence of short duration. E. Becquerel has devised an apparatus for doing this, a _phosphoroscope_. It consists of revolving disks with holes in them between which the object to be examined is placed. The holes are so arranged that the object is first illuminated and then completely cut off from light. The observer looking at it through another hole sees it at the moment it is not illuminated and can thus tell if it is phosphorescing. By determining the rate of revolution of the disks it is easy to calculate how long the phosphorescence persists. While relatively few solids phosphoresce after exposure to light at ordinary temperature a large number of these acquire the property at the temperature of liquid air. Included in the list are such biological products as urea, salicylic acid, starch, glue and egg shells. The temperature also affects the wave-length and hence the color of the light given off. Usually the higher the temperature the shorter the wave-length, but in the case of some bodies (SrS) the wave-lengths become longer at the higher temperature.