Combustion

(kom-büst'yun.) [From L. comburo, I consume.] (Chem.) When substances combine chemically, and the combination is attended by the evolution of light and heat, the phenomenon is called C. All ordinary C. is the union of an inflammable body with oxygen gas, the most familiar example of which is found in the burning of coal in a fireplace. As other forms of C. we have metals burning in chlorine, or the vapor of bromine. Substances, like oxygen, which combine with inflammable, bodies attended by the phenomenon of C., are called supporters of C.; while the substances burnt, such as coal, are called combustibles. The term slow C., which is sometimes used in such cases as the gradual oxidation of moist phosphorus, is very inappropriate, and should always be replaced by slow oxidation, or slow chemical union; because C. is a more or le violent action, accompanied by the production of intense light and heat. When carbon is burnt in oxygen gas, we have an example of C.; but when the electric arc pa es between two carbon points placed in a vacuum, we have an example of ignition. According to a theory, which has received considerable support, the heat produced during chemical combustion is caused by the direct conversion of motion into heat. Thus, the C. of carbon in oxygen is said to be due to the clashing of carbon and oxygen atoms, which rush together under the influence of the force of chemical affinity with an enormous velocity, and when they come into collision their motion of translation is transmuted into that kind of vibratory Comedy, motion which (kom'e-de.) we call heat. [Gr. komōdia, literally, a village song, because the first comedies consisted of rustic dialogues.] (Lit.) A dramatic representation of the light, humorous, and pleasant kind, particularly intended to ridicule the foibles and follies of men. Scaligerdefines C. to be a dramatic poem, representing the busine of life, whose event is fortunate and style familiar. With us, C. is distinguished from farce, inasmuch as the former represents nature as she is, while the latter distorts and overcharges her. Comet, (kuüm'it.) [Gr. komētēs, from kome, hair.) (Ast.) A celestial object presenting a nebulous aspect, but traversing the interstellar spaces, and becoming known to us by pa ing within the limits of the sun's attraction. Many of them belong to the solar system, travelling in closed paths around the sun. Although the idea that comets may travel in periodic orbits around the sun had suggested itself to the ancients, we owe to Newton the first enunciation of this theory. He founded it upon the calculations he had applied to the motions of the great C. of 1680. The theory can hardly be said to have been proved, however, until the time of Hailey's researches into the motions and periodic re turns of the C. which bears his name, or perhaps even until the date of the first return of this C. in accordance with Halley's predictions. A C. usually presents the appearance of a coma, or haze of light surrounding a somewhat bright nucleus. As the C. approaches the sun the haze of light generally grows elongated, and when the C. is a large one, traces begin to be seen which indicate the approaching formation of a tail. A certain appearance of streakine in the C's light usually precedes the formation of the tail. The direction of the tail is nearly always from the sun. It grows longer and brighter as the C. approaches perihelion. After perihelion pa age many comets are greatly changed in appearance. Some are brighter and more striking than they were before perihelion pa age; while others are shorn of a large proportion of their splendor. The only feature which belongs to all comets is the coma. Many comas have no nucleus, and quite a large proportion have no tail; on the other hand, some comets have more than one tail. The real dimensions of comets must, in many cases, be regarded as inconceivably vast. The C. of1680, one of the most celebrated of modern times, had a tail that was calculated at twenty millions of leagues immediately after its perihelion pa age, and yet it was emitted in two days. It subsequently became more than twice as long. The matter of these tails must be of incalculable tenuity, and it is supposed that on account of the distance to which they are sent, much of it can never be re-absorbed. Even the heads of many comets are composed of matter of extreme thinne , for a star of the fifth magnitude was observed through the densest part of one without undergoing any diminution of lustre. We know so little respecting the physical condition of comets that it would be hazardous to speculate at present concerning their real nature. A theory of great ingenuity, and (what is novel in this branch of speculation) founded on physical experiments which really seem to have some bearing on the subject, has lately been put forward by Profe or Tyndall, who is disposed to regard the tails of comets as resulting from the formation of a species of actinic cloud by the action of the solar rays, after their character has been altered during their pa age through the comet's head. At present, however, it is difficult to say whether such a theory is well or ill founded. [s. 210]