Some even go so far as to assert that all minerals containing a fixed acid are capable of becoming phosphorescent by insolation or other means. Fused nitrate of calcium and petrifactions are also rendered luminous by this means. It is a fact worth noting that insolation affects the phosphorescence best in badly-conducting minerals.

Phosphorescence has been noticed, too, in the change from the amorphous to the crystalline state, and separation of crystals from a solution. Another source of this light, and one not often noticed, is that caused by mechanical means, either percussion or friction; the phosphorescence, however, in this case, lasts only so long as the disturbing influence is at work. Among the substances that are mentioned as affected by this agency, the most characteristic is adularia, a transparent variety of potash felspar (orthoclase) which is found in fine crystals in Cornwall, and also at St. Gothard, in the Alps. This remarkable mineral, when struck so as to split, shows at each crack a streak of light which may last some little time, and when ground in a mortar has the property of appearing to be on fire. Quartz, fluorspar, and rock-salt, also exhibit this property when pounded, but in a far less degree than adularia. In the great majority of cases, the duration of the phosphorescence in natural bodies is extremely short; but, nevertheless, Becquerel invented a most ingenious instrument, which he called a " phosphoroscope," which will measure the length of the existence of the most short-lived phosphorescent flash.

An interesting and remarkable discovery was made by Beccari, of Turin, who found that a phosphorescent body gave out in many cases the light to which it had been subjected. This ingenious philosopher also discovered that snow could be rendered slightly luminous by insolation. Some specimens of zinc-blende also give out phosphorescent light, even with so slight an exciting cause as the friction of a feather; and also some kinds of marble show, when heated, a yellowish phosphorescence. (G. R. T., in Eng. Mech.)

The luminosity of minerals has an obvious practical value in the case of such substances as can be conveniently applied in the form of a paint to surfaces which are alternately exposed to light and darkness, such exposed surfaces emitting at one time the light which they have absorbed at another. Familiar illustrations are street plates, buoys, and interiors.of railway carriages having to traverse many tunnels. The light absorbed may be either daylight or powerful artificial light. With this object, several compositions are prepared under the generic name of luminous paints. They are chiefly as follows:

(1) Balmain's

This consists of a phosphorescent substance introduced into ordinary paint. The phosphorescent substance employed for the purpose is a compound obtained by simply heating together a mixture of lime and sulphur, or substances containing lime and sulphur, such as alabaster, gypsum, etc, with carbon or other agent to remove a portion of the oxygen present; or by ating lime in a vapour containing sulphur. In applying this phosphorescent powder, the best results are obtained by mixing it with a colourless varnish made from mastic and turpentine; drying oils, gums, pastes, sizes, etc, may, however, also be used.

(2) A French compound. 100 lb. of a carbonate of lime and phosphate of lime produced by the calcination of sea-shells, and especially those of the genus Tridacna and the cuttle-fish bone, intimately mixed with 100 lb. of lime rendered chemically pure by calcination, 25 lb. of calcined sea-salt, 25 to 50 per cent, of the whole mass of sulphur, incorporated by the process of sublimation, and 3 to 7 per cent, of colouring matter in the form of powder composed of mono-sulphide of calcium, barium, strontium, uranium, magnesium, aluminium, or other mineral or substance producing the same physical appearances, i.e. which, after having been impregnated with light, becomes luminous in the dark. After having mixed these five ingredients intimately, the composition obtained is ready for use. In certain cases, and more specially for augmenting the intensity and the duration of the luminous effect of the composition, a sixth ingredient is added in the form of phosphorus reduced to powder, which is obtained from seaweed by the well-known process of calcination.

As to proportion, it is found that the phosphorus contained in a quantity of seaweed, representing 25 per cent, of the weight of the composition formed by the five above-named ingredients, gives very good results.

The phosphorescent powder thus obtained and reduced into paste by the addition of a sufficient quantity of varnish, such as copal, may serve for illuminating a great number of objects, by arranging it in more or less thick cpatings, or by the application of one or more coatings of the powder incorporated in the varnish, or by varnishing previously and sprinkling the dry powder upon the varnish. The amount of powder applied should not exceed the thickness of a thin sheet of cardboard.

The dry phosphorescent powders are also converted into translucent flexible sheets of unlimited length, thickness, and width, by mixing them with about 80 per cent, of their weight of ether and collodion in equal parts in a close vessel, and rolling the product into sheets with which any objects may be covered which are intended to be luminous in the dark. The powders may also be intimately mixed with stearine, paraffin, rectified glue, isinglass, liquid silex, or other transparent solid matter, in the proportion of 20 to 30 per cent, of the former with 50 to 80 per cent, of either of these substances, and this mass is then reduced into sheets of variable length, width, and thickness, according to their intended applications. A luminous glass is also manufactured by means of the powders by mixing them in glass in a fused state in the proportions of 5 to 20 per cent, of the mass of glass. After the composition has been puddled or mixed, it is converted into different articles, according to the ordinary processes; or after the manufacture of an object still warm and plastic, made of ordinary glass, it is sprinkled with the powders, which latter are then incorporated into the surface of the article by pressure exerted in the mould, or in any other suitable way.

It has been observed after various trials that the passage of an electric current through the different compositions augments their luminous properties or brilliancy to a great extent; this peculiarity is intended to be utilized in various applications too numerous to describe; but of which buoys form a good example. The current of electricity is furnished by plates of zinc and copper mounted on the buoy itself, when the latter is used at sea; but in rivers and fresh-water inlets the battery will be carried in the interior of the buoy. To secure the full effect, 10 to 20 per cent, of fine zinc, copper, or antimony dust is added to the phosphorescent powder described.

(3) Take oyster-shells and clean them with warm water; put them into the fire for 1/2 hour; at the end of that time take them out and let them cool. When quite cool, pound them fine, and take away any grey parts, as they are of no use. Put the powder in a crucible with alternate layers of flowers of sulphur. Put on the lid, and cement with sand made into a stiff paste with beer. When dry, put over the fire and bake for an hour. Wait until quite cold before opening the lid. The product ought to be white. You must separate all grey parts, as they are not luminous. Make a sifter in the following manner: Take a pot, put a piece of very fine muslin very loosely across it, tie around with a string, put the powder into the top, and rake about until only the coarse powder remains: open the pot, and you will find a very small powder. Mix it into a thin paint with gum water, as two thin applications are better than one thick one. This will give paint that will remain luminous far into the night, provided it is exposed to the light during the day.

(4) Sulphides of calcium, of barium, of strontium, etc, give phosphorescent powders when duly heated. Each sulphide has a predominant colour, but the temperature to which it is heated has a modifying effect on the colour. Calcine in a covered crucible, along with powdered charcoal, sulphate of lime, sulphate of barytes, or sulphate of strontia; there is produced in each case a greyish white powder, which, after exposure to strong light (either sunlight or magnesium light), will be phosphorescent, the colour depending on the sulphate used and the degree of heat employed.

(5) Five parts of a luminous sulphide of an alkaline earth, 10 of fluorspar, cryolite, or other similar fluoride, 1 of barium borate; powdered, mixed, made into a cream with water, painted on the glass or stone article, dried, and fired in the usual way for enamels. If the article contains an oxide of iron, lead, or other metal, it must be first glazed with ground felspar, silica, lime phosphate, or clay, to keep the sulphur of the sulphide from combining with the metal. The result is an enamelled luminous article. (Heaton and Bolas.)

(6) Boil for 1 hour 2 1/4 oz. caustic lime, recently prepared by calcining clean white shells at a strong red heat, with 1 oz. pure sulphur (floured) and 1 qt. soft water. Set aside in a covered yesssel for a few days; then pour off the liquid, collect the clear orange-coloured crystals which hare deposited, and let them drain and dry on bibulous paper. Place the dried sulphide in a clean graphite crucible provided with a cover. Heat for 1/2 hour at a temperature just short of redness, then quickly for about 15 minutes at a white heat. Remove cover, and pack in clay until perfectly cold. A small quantity of pure calcium fluoride is added to the sulphide before heating it. It may be mixed with alcoholic copal varnish. (Boston J I. Chem.)