Another principle of heat is _Convection_, by which water, air, and gases are warmed. This is, literally, the process of _conveying_ heat from one portion of a fluid body to another by currents resulting from changes of temperature. It is secured by bringing one portion of a liquid or gas into contact with a heated surface, whereby it becomes lighter and expanded in volume. In consequence, the cooler and heavier particles above pressing downward, the lighter ones rise upward, when the former, being heated, rise in their turn, and give place to others again descending from above. Thus a constant motion of currents and interchange of particles is produced until, as in a vessel of water, the whole body comes to an equal temperature. Air is heated in the same way. In case of a hot stove, the air that touches it is heated, becomes lighter, and rises, giving place to cooler and heavier particles, which, when heated, also ascend. It is owing to this process that the air of a room is warmest at the top and coolest at the bottom. It is owing to this principle, also, that water and air can not be heated by fire from above. For the particles of these bodies, being non-conductors, do not impart heat to each other; and when the warmest are at the top, they can not take the place of cooler and heavier ones below. Another principle of heat (which it shares with light) is _Radiation_, by which all things send out heat to surrounding cooler bodies. Some bodies will absorb radiated heat, others will reflect it, and others allow it to pass through them without either absorbing or reflecting Thus, black and rough substances absorb heat, (or light,) colored and smooth articles reflect it, while air allows it to pass through without either absorbing or reflecting. It is owing to this, that rough and black vessels boil water sooner than smooth and light-colored ones. Another principle is _Reflection_, by which heat radiated to a surface is turned back from it when not absorbed or allowed to pass through; just as a ball rebounds from a wall; just as sound is thrown back from a hill, making echo; just as rays of light are reflected from a mirror. And, as with light, the rays of heat are always reflected from a surface in an angle exactly corresponding to the direction in which it strikes that surface. Thus, if heated are comes to an object perpendicularly--that is, at right angles, it will be reflected back in the same line. If it strikes obliquely, it is reflected obliquely, at an angle with the surface precisely the same as the angle with which it first struck. And, of course, if it moves toward the surface and comes upon it in a line having so small an angle with it as to be almost parallel with it, the heated air is spread wide and diffused through a larger space than when the angles are greater and the width of reflection less. [Illustration: Fig. 31.] [Illustration: Fig. 32.] [Illustration: Fig. 33.]