MERITS OF LIGHTING BY INCANDESCENT MANTLES.--It has already been shown that acetylene bases its chief claim for adoption as an illuminant in country districts upon the fact that, when consumed in simple self- luminous burners, it gives a light comparable in all respects save that of cost to the light of incandescent coal-gas. The employment of a mantle is still accompanied by several objections which appear serious to the average householder, who is not always disposed either to devote sufficient attention to his burners to keep them in a high state of efficiency or to contract for their maintenance by the gas company or others. Coal-gas cannot be burnt satisfactorily on the incandescent system unless the glass chimneys and shades are kept clean, unless the mantles are renewed as soon as they show signs of deterioration, and, perhaps most important of all, unless the burners are frequently cleared of the dust which collects round the jets. For this reason luminous acetylene ranks with luminous coal-gas in convenience and simplicity, while ranking with incandescent coal-gas in hygienic value. Very similar remarks apply to paraffin, and, in certain countries, to denatured alcohol. Since those latter illuminants are also available in rural places where coal-gas is not laid on, luminous acetylene is a less advantageous means of procuring artificial light than paraffin (and on occasion than coal-gas and alcohol when the latter fuels are burnt under the mantle), if the pecuniary aspect of the question is the only one considered. Such a comparison, however, is by no means fair; for if coal- gas, paraffin, and alcohol can be consumed on the incandescent system, so can acetylene; and if acetylene is hygienically equal to incandescent coal-gas, it is superior thereto when also burnt under the mantle. Nevertheless there should be one minor but perfectly irremediable defect in incandescent acetylene, viz., a sacrifice of that characteristic property of the luminous gas to emit a light closely resembling that of the sun in tint, which was mentioned in Chapter 1. Self-luminous acetylene gives the whitest light hitherto procurable without special correction of the rays, because its light is derived from glowing particles of carbon which happen to be heated (because of the high flame temperature) to the best possible temperature for the emission of pure white light. The light of any combustible consumed on the "incandescent" system is derived from glowing particles of ceria, thoria, or similar metallic oxides; and the character or shade of the light they emit is a function, apart from the temperature to which they are raised, of their specific chemical nature. Still, the light of incandescent acetylene is sufficiently pleasant, and according to Caro is purer white than that of incandescent coal-gas; but lengthy tests carried out by one of the authors actually show it to be appreciably inferior to luminous acetylene