Of several objects in a cold room, some feel much colder to the touch than others, thus iron, marble, oil cloth, and earthenware will feel colder than woolen cloth, carpet, feathers, or paper. The first four objects feel cold because they are conductors, and conduct the heat away from the hand rapidly. The other substances named are non-conductors and hence remove heat from the hand less rapidly, and therefore do not feel so cold. In a similar way, if several hot objects are touched by the hand, the good conductors are the ones which will burn one most quickly by conducting heat rapidly to the hand. The non-conductors, however, will rarely burn one. Why are the handles of hot utensils often made of non-conducting materials such as wood, cloth, asbestos, etc.? [Illustration: FIG. 134.--Cross-section of the vacuum flask in a Thermos bottle.] =153. Radiation= is the method by which heat comes to us from the sun across space containing no tangible matter. It is also the method by which heat gets to us when we stand near a fire. Everyone has noticed that this heat is cut off by holding an object between the person and the fire. This fact indicates that radiant heat travels in _straight_ lines. _The radiation of heat_ is believed to be accomplished by means of waves in a medium called _ether_, which is invisible and yet pervades everything. Three of the most important characteristics of radiation are _first, heat is transferred by radiation with the speed of light_, or 186,000 miles per second. This fact is shown by the cutting off of both the sun's heat and light at the same instant during an eclipse of the sun. _Second, radiant heat[I] travels in straight lines_, while other modes of transferring heat may follow irregular paths. The straight line motion of radiant heat is shown by its being cut off where a screen is placed between the source of heat and the object sheltered. _Third, radiant heat may pass through an object without heating it._ This is shown by the coldness of the upper layers of the atmosphere and also by the fact that a pane of glass may not be heated appreciably by the heat and light from the sun which passes through it. [I] Radiant heat is really _radiant energy_ and becomes heat when it is absorbed by a body. When radiant energy falls upon any object it may be (a) _reflected_ at the surface of the object, (b) _transmitted_ through the substance, (c), absorbed. All three of these effects occur in different degrees with different portions of the radiation. _Well-polished surfaces are good reflectors._ Rough and blackened surfaces are _good absorbers_. Transparent objects are those which transmit light well, but even they absorb some of the energy.