Page:Popular Science Monthly Volume 17.djvu/726

708 The phosphorescent power of barium sulphide was discovered in 1675. Canton, in 1761, discovered the so-called phosphorus which bears his name by calcining oyster shells with sulphur so as to form the calcium sulphide, the most remarkable of phosphorescent substances. Its luminosity appears to be permanent, for Professor Heaton, in a lecture describing the qualities of the luminous paint, delivered before the Society of Arts, on the 11th of March last, exhibited a specimen of it which had been sealed in glass by Canton himself in 1764, and which still glowed. The power is attributed to a property which the substances possessing it have of absorbing rays and afterward emitting them with an increased wave-length, as is remarkably shown by quinine, which shines after having been exposed to the ultra-violet part of the spectrum, and converts invisible actinic into visible light-rays. The late Mr. Balmain succeeded in producing from a compound of lime and sulphur a constant and very powerful phosphorescent substance, which he patented and applied as a paint; articles coated with it become luminous after exposure to the light and retain their glow for a considerable length of time. The sensitiveness of this substance was shown during Professor Heaton's lecture by passing electric sparks in front of a card painted with it which had been previously kept in darkness. Each spark impressed its image on the card, and made it luminous. Even a lucifer-match struck in front of a dark pane produced a visible effect on the paint. The highest effect is produced by the violet and ultra-violet rays. The red and yellow rays do not add to the luminosity; in fact, they diminish it when they are allowed to continue to act for a considerable time. This is owing to their calorific effect, which, though it may stimulate the light for a while, in the end causes it to disappear more rapidly. A short exposure of the paint to ordinary daylight is sufficient to produce a high degree of illumination, the amount and duration of which will depend considerably on the quantity and quality of the light and on other conditions. When the paint has been exposed to the intense light of the sun or of burning magnesium, a good deal of the brilliancy disappears quickly, but after that the fading is very slow; and it may be said that a more or less useful light will remain through the length of an ordinary winter's night. In one case, Professor Heaton was just able to see the dial of a watch by the light emitted from a card which, after having been exposed to daylight of moderate intensity for two hours, had been in total darkness for twenty-six hours. The paint appears to be of satisfactory durability as against all weathers and the action of seawater. The useful purposes to which it may be applied are almost innumerable. Clock-faces painted with it will show the time; match-boxes can be found, all through the night; the roofs of railway-cars will light passengers through tunnels; buoys in harbors and channels, and life-buoys, can be made visible; ships may show themselves to each other in the darkness; the diver, painting his dress with it, may be his own lantern and carry enough daylight with him to enable him to work under water at a considerable advantage. Its application to use in powder-magazines and coal-mines, and wherever fire-lights are dangerous, may be considered as among the things that are practicable. The manufacturers of the paint, Messrs. Ihlee & Home, of Aldermanbury, London, say that it now costs twenty-eight shillings, or about seven dollars, a pound, and that a pound of it will paint about twenty-eight square feet. Two years ago they held it at five guineas a pound, but they have been able to lower the price gradually, and hope ultimately that it may be afforded at less than the cost of white-lead.

Is South America rising or sinking?—The question whether the South American Continent is sinking or not is one on which considerable difference of opinion still exists. Professor Orton several years ago expressed the belief that the barometric observations of the heights of the principal mountains, which have been continued through more than a hundred years, afforded evidence of a gradual sinking, and this opinion has prevailed extensively. Professor Agassiz believed that the eastern coast was sinking while the western coast was rising, and Darwin infers, from the discovery of the remains of an ancient civilization on lands that are now too high for the development of human life, that the land is