DISSOLVING VIEWS.—See Magic Lantern.
DISTANCE.—The extreme boundary of view in a picture, that part which appears farthest from the eye. In perspective the point of distance is that point of a picture where the visual rays meet. The middle distance is the central part of the picture, and the line of distance is a straight line drawn from the eye to the principal point on the plane. (See also Aerial Perspective.) .
DISTILLATION.—The process of applying heat to a liquid, or it may be a solid, in order that certain constituents may pass away in vapor, and by suitable arrangements be obtained in liquid form. When the vapor does not condense as a liquid, but only as fine dust or flour, the process is called The apparatus for artificial distillation consists of three parts—the still, the condenser, and the receiver. The still is usually made of glass, copper, iron, or earthenware, according to the na ture of the substances to be placed in it. In experimental chemical work glass is almost the only material admissible, while in the preparation of alcohol from grain copper stills are commonly employed. The condenser is made in an infinity of forms, the object being to condense the vapors disengaged from the still rapidly and as effectually as possible. For this purpose it is important that the condenser shall expose a larger surface to the cooling medium—water or air. Whatever the form the principle is the same, in that the hot vapors pass through a tube or vessel surrounded with cold water, which, running in a constant stream, passes away more or less warm after it has done its work. The receiver merely consists of a suitable vessel to receive the distillate.
There are various forms of distillation. Those best known are fractional distillation, destructive distillation, and distillation in vacuo.
Fractional mixture of liquids having different boiling points is placed in a still, and heat is applied. If the temperature be raised slowly, and especially if the vapor requires to rise through a high tube before passing into the condenser, it will be found that the more volatile liquids pass over first in a state of purity and while the others follow as the heat is increased. If the receiver be frequently changed a series of fractions are obtained corresponding to the different substances in the still. For example, if a mixture of glycerine, water, alcohol, chloroform, and ether were distilled, the ether would distil first, then the chloroform, next the alcohol, and lastly the water, while nearly all the glycerine would be left behind. This, then, is fractional distillation.
Destructive Distillation is accomplished by heating non-volatile organic bodies in a retort.
Distillation in water is heated in a kettle it eventually boils, and if a thermom eter be plunged in it will register 212 deg. Fahr. If the barometer is low, however, the temper ature of the boiling water will be somewhat lower than 212 deg. Fahr., and if the barometer is very high the temperature is also high. On the summit of Mont Blanc water boils at i8o deg., while in a vessel from which the air has been removed by an air pump it continues to boil even when the temperature falls down below freezing point. These substances which are injured by heat are distilled in this manner in vacuo.