been suggested that "living lamps" made from these bacteria could be used to advantage in coal mines and powder magazines, since as they emit no appreciable heat, they would be absolutely without danger of producing an explosion. They would certainly furnish a nice cool light for use in summer; the author would very much like to have one here in his little study now, in place of the Welsbach which is engaged in turning some 98 per cent, of its expended energy into heat instead of light, after a day that has shown 90° F. However, such lamps would probably not do for anything like general illumination, even if the intensity were great enough, for in their light any color effects beyond a very limited range would he impossible. Color considerations would not be a considerable factor in mines and mills, however, so this does not interfere with this application.
It is more reasonable, however, to consider that these luminous forms merely point to what is possible in the way of efficient light—to serve as the goal to which all effort in the improvement of light-efficiency must strive. This must be the ideal—light without heat.