under the shock of a fulminate cap much in the same way that nitroglycerine explodes, the hydrocarbon being resolved into carbon and hydrogen. Such detonation is violent in proportion to the pressure, and appears only likely to be comparable in destructiveness with a nitroglycerine explosion when the acety-lene gas is compressed to a liquid, which at the freezing point of water requires a pressure of about 725 lb. on the square inch. A feeble passage of detonation all through the gas has, however, been noticed under a total pressure of two atmospheres ; that is to say, a pressure of about 14 lb. on the square inch, as an engineer would ordinarily reckon it, or counting only the addi-tional pressure beyond the normal pressure. In January, 1895, the use of acetylene as an illuminant was first discussed in the photographic papers, and'about a year later calcium carbide was quite easily obtainable in London. Photographers began to experiment, and automatic generators, on the principle of Dober-einer's hydrogen lamp, became common. Great hopes were entertained that the use of acetylene would revolutionise the optical lantern, and that this gas would prove of great practical value for portraiture by artificial light. These hopes have not been realised, although some other uses have been made of the acetylene light.
Achromatic (a, prefix signifying negation, and coloured), when applied to a lens, signifies that it has been partially corrected for chromatic aberration, and that the images projected by it are unaccompanied by fringes of various colours. The correction is usually effected by combining two glasses having differing refractive powers, as, for instance, a convex crown-glass lens with a concave flint-glass, or by enclosing a flint meniscus between two concavo-convex. There are several methods, but the latter is the usual method employed for the rapid class of lenses now so much in vogue. (See Lens.) A more completely corrected lens is styled apochromatic. (For the theory involved in rendering lenses achromatic, see Decomposition of Light.)
Acidity, or sourness. A frequent property of acids, though not invariably a characteristic (see below); moreover, many sub-stances which are not acids within the usual definition are sour. Paper stained with blue litmus is a common test for acidity,