Page:The Readable Dictionary.djvu/45

Rh  in the degree or intensity of the color; as, a pale red, a pale blue.

Pale, when applied to the countenance, denotes an absence of the freshness which is indicative of ordinary health.

is an excess of paleness caused by protracted sickness, hunger, or fatigue.

is a ghastly paleness indicative of extreme prostration of the vital powers by protracted sickness or want of food.

To is to apply coloring matter mixed with oil or water to the surface of any thing.

or are colored substances used in painting.

To is to color substances by immersing them in a watery solution of some coloring matter.

To is to change the color by the application of a coloring matter that enters the pores of the substance to which it is applied.

To is to whiten either by sunlight, or by exposure to the influence of certain vapors.

To is a term applied to certain processes of whitening. (Fr., blanc, white.)

HEAT is the cause of the sensation which we call warmth.

1.—Two theories have been held by philosophers in regard to the nature of heat, some looking on it as a material fluid, and others maintaining that it depends on vibrations in the universal ether that fills all space, and pervades the pores even of the most dense bodies.

Those who hold the former theory, call the element of heat caloric. They say that caloric exists in two states; first, that of latent (hidden) caloric, in which the caloric is intimately combined with the substance of bodies, and does not affect the thermometer or the sensibility; and second, that of free caloric, in which state the caloric affects the thermometer and produces sensation.

Radiant caloric is a modification of free caloric, and is subject to the same laws that regulate the radiation and reflection of light.

Those who hold the second theory suppose that heat is closely allied to light, and that the rays of heat differ from the luminous rays of any particular color, in the same way that the differently colored rays differ among themselves. Heat and light, they say, depend on undulations in the same elastic ether. The undulations which produce the phenomenon of blue light are supposed to be smaller and more rapid than those which give rise to the phenomenon of yellow light. The undulations from which yellow light results, are again quicker and more limited in their range of motion than those from which red light results; while those which give rise to heat are less frequent, and traverse a wider space than do any of the undulations on which the colored rays depend.

2.—Caloric, or the principle of heat, is the cause of fluidity. Were it not for this principle, all substances—even air—could exist only as solids. It is supposed that a due degree of heat would convert the most refractory solids into liquids, and that under some higher temperature these liquids would be changed into gases.

3.—The sources of heat are the sun, chemical action, and mechanical force.

1. When light proceeds directly from the sun, the rays of light are combined with those of heat; but when solar light is reflected from the moon, it contains no appreciable heat.

2. Whenever substances combine with each other under the influence of chemical affinity, heat is evolved, but not always in such a degree as to affect the thermometer to any appreciable extent; but if substances combine rapidly and with great energy, the heat generated is sometimes very intense, as when water combines with fresh-burned lime.

3. The modifications of mechanical action by which heat may be produced, are friction, (or rubbing,) percussion, (or striking,) and condensation, (or pressing together.)

1st. Friction.—Two sticks of dry wood may be ignited (or set on fire) by rubbing them forcibly together.

2d. Percussion.—If a small piece of iron be placed on an anvil, and be subjected for a few moments to quickly-repeated strokes of a hammer, it will become hot.

3d. Condensation.— If a piece of tinder be placed at the bottom of a tube to which a piston