Note 44 — Chapter XI · HUMPHRY AND HIS “WONDERFUL LAMP.”

A few flowers have also the property of emitting light at ordinary temperatures. Among these may be cited the tube-rose, nasturtium, and marigold, which occasionally give out flashes of light on a warm summer’s evening. This, probably, arises from the combination of the oxygen of the atmosphere with the vapour of the volatile oils upon which the perfumes of flowers are known to depend. Again, other substances emit light during the act of crystallization. This phenomenon is most distinctly observed during the gradual deposit of arsenious acid, when dissolved in hot hydrochloric acid (spirits of salts). In a dark place each crystal, as it is formed, may be seen to emit a spark of light; and on shaking the flask, so many crystals are sometimes suddenly produced, that vivid flashes become perceptible. The cause of this phenomenon is probably dependent on the fixation of oxygen by the arsenic at the moment of precipitation.