_s. d._ Electricity Per unit, 3-1/2d. " Incandescent, 1 2 " Arc, 0 3-3/4 Coal-gas Flat flame, 1 6 " Incandescent, 0 2-1/4 " " high pressure, 0 1-3/4 Oil Lamp (oil at 8d. per gall.), 0 7-1/4 " Incandescent lamp, 0 2-1/4 " Kitson lamp, 0 1 Petroleum, therefore, at present comes in a very good first in England. The system that we have noticed at some length has been adapted for lighthouse use, as it gives a light peculiarly fog-piercing. It is said to approximate most closely to ordinary sunlight, and on that account has been found very useful for the taking of photographs at night-time. The portability of the apparatus makes it popular with contractors; and the fact that its installation requires no tearing up of the streets is a great recommendation with the long-suffering public of some of our large towns. Another very powerful light is produced by burning the gas given off by carbide of calcium when immersed in water. _Acetylene_ gas, as it is called, is now widely used in cycle and motor lamps, which emit a shaft of light sometimes painfully dazzling to those who have to face it. In Germany the gas is largely employed in village streets; and in this country it is gaining ground as an illuminant of country houses, being easy to manufacture--in small gasometers of a few cubic yards capacity--and economical to burn. Well supplied as we are with lights, we find, nevertheless, that savants are constantly in pursuit of an _ideal_ illuminant. From the sun are borne to us through the ether light waves, heat waves, magnetic waves, and other waves of which we have as yet but a dim perception. The waves are commingled, and we are unable to separate them absolutely. And as soon as we try to copy the sun’s effects as a source of heat or light we find the same difficulty. The fire that cooks our food gives off a quantity of useless light-waves; the oil-lamp that brightens one’s rooms gives off a quantity of useless, often obnoxious, heat. The ideal illuminant and the ideal heating agent must be one in which the required waves are in a great majority. Unfortunately, even with our most perfected methods, the production of light is accompanied by the exertion of a disproportionate amount of wasted energy. In the ordinary incandescent lamp, to take an instance, only 5 or 6 per cent. of the energy put into it as electricity results in light. The rest is dispelled in overcoming the resistance of the filament and agitating the few air-molecules in the bulb. To this we must add the fact that the current itself represents but a fraction of the power exerted to produce it. The following words of Professor Lodge are to the point on this subject:--