150 ELEMENTS OF LABORATORY WORK 



CHAPTER VIII 



OBSERVATIONS WHICH LEAD TO THE THEORY THAT SPACE IS 

 FILLED WITH A MEDIUM, THE ETHER, BY MEANS OF WHICH 

 CERTAIN MODES OF MOTION ARE CONVEYED FROM ONE PORTION 

 OF MATTER TO ANOTHER 



96. On Kadiation or Rectilinear Propagation of Light and 

 Temperature. We have already noticed that thermal equili- 

 brium may come about, without material contact, by means of 

 radiation a process which takes place independently of any 

 material medium, or, at any rate, of matter as we have learnt 

 to observe it. The effects of radiation may be detected in any 

 direction in space, except so far as they are intercepted by 

 material bodies. 



Radiating matter may be recognised either by thermal or 

 luminous effects ; that is, we may receive from it the sensation 

 of warmth through our skin, or through our eyes the sensation 

 of light. Although these properties may co-exist, a body which 

 causes a change of temperature in neighbouring bodies does 

 not necessarily affect our eyes. 



If a body, e.g. a piece of iron, be raised to a high tempera- 

 ture, it will be found that the gradual diminution of thermal 

 radiation, as marked by a neighbouring thermometer, corre- 

 sponds with a change from whiteness to a yellow, and then red, 

 glow ; while considerable thermal radiation will go on after 

 the body has ceased to emit any light of its own. The same 

 phenomena occur in reverse order as the temperature of a 

 body rises. It is important to distinguish between the sensa- 

 tion of light and its cause, and the sensation of warmth and 

 its cause, and also to remember that temperature is a condition 

 of matter. We are now primarily concerned in finding out 



