CHAPTER XIV. 

 GENERAL ACCOUNT OF RADIATION. 



Radiant Energj Radiometers Radiant Energy and Light resemble each other 

 Light is Radiant Energy to which the eye is sensitive Radiant Energy has a 

 much greater range of Wave-Length than Light Radiometers only measure 

 Energy Slreams and do not indicate Quality Comparison of Emissive Powers 

 Radiation of different Wave Lengths Comparison of Absorptive Powers 

 Comparison of Reflecting Powers Diffusion General results Radiating and 

 Absorbing Powers vary together Illustrations Transparency and Opacity- 

 Radiation and Absorption by Gases and Vapours. 



Radiant Energy. When heat is received from a hot fire through 

 an intervening space of air, the air is not permanently affected by the 

 passage of the energy through it ; there is no continuous downward slope 

 of temperature from the fire to the receiver, and frequently, as on a cold 

 winter's day, the air may be far colder than the receiver. The energy of 

 the hot body spreads out on all sides from the body as a centre if unim- 

 peded by obstacles, and it is therefore said to be radiated. The process 

 is quite distinct from conduction, a good conductor like copper being as 

 efficient an obstacle to the propagation of the energy as a bad conductor 

 like wood, while air (as we have seen, an exceedingly poor conductor), 

 allows the energy to pass freely. Before leaving the radiating body the 

 energy radiated is evident as heat, and on reaching the receiver it is 

 again evident as heat ; but since in transit it does not make itself 

 evident by warming the medium, we must ascribe to it a form distinct 

 from heat which we term radiant energy. 



In this chapter we shall investigate the nature of radiant energy, 

 and discuss its emission from sources and its absorption and reflection at 

 surfaces upon which it falls. 



Radiometers. It is necessary, of course, to have some instrument 

 to receive, and indicate the rate of reception of, the energy arriving at a 

 given point. We may term such an instrument a radiometer, though 

 that name is frequently applied to one particular instrument, Crookes's 

 Radiometer, described on p. 150. 



When the rate at which the energy is arriving is large, we can 

 detect it directly by the temperature sense, that sense which tells us 

 when the skin is warmed or cooled. But obviously the temperature 

 sense can only give us qualitative indications, and is no more suited for 

 radiation measurements than for measurements of temperature. 



Differential Thermometer. Leslie, a pioneer in research on 

 radiation, used the differential thermometer, which consists of a U tube 

 with a bulb at each end of the U. A small quantity of coloured liquid 

 is introduced before the bulbs are sealed off, and the air in the two bulbs 



