Apparent Radiation of Cold. 237 

 have their number of radiatory points directly as the square of their 

 distances. Therefore the number of rays received from them by 

 the thermometer, will be equal, whilst the surfaces possess the 

 same temperature and radiating power, provided no rays are inter- 



their influence will be as their apparent magnitude ; if this be con- 

 stant, their calorific or refrigerant influence varies with their tern- 

 'perature. Therefore, whilst a cold body in a warm chamber sup- 

 plies a thermometer in its vicinity, with some calorific emanations, 

 it intercepts a greater number which would have been received 

 from the warmer walls of the apartment, and thus prevents that 

 equivalent return which the thermometer had before received for 

 its own radiation. For it subtends the same angle at the thermo- 

 meter, as the portion of wall whose rays it intercepts, and it is of in- 

 ferior temperature. Now it is evident, that a large cold body will 

 at the same distance intercept rays still more numerous, and in 

 still greater excess, above those which it supplies, and consequent- 

 ly that it will exert a cooling agency still more powerful. 



An other and more plausible objection to this theory, [vide Mur- 

 ray's Chem. vol. 1, and Edin. Encyclop. Art. "Cold,"] is founded 

 on the following experiment. A cubical vessel, having one of its 

 sides covered with lampblack, and another lefl with a metallic sur- 

 face, was rilled with a frigorific mixture and placed in the focus of 

 a metallic reflector, having the surface of a parabolic conoid. A 

 thermometer was placed in the focus of the generating parabola of 

 an equal and similar mirror placed opposite the former, at the dis- 

 tance of some feet, with their axes coincident. The thermometer 

 suffered the greatest reduction of temperature when the painted 

 side of the cold body was presented towards that mirror in whose 

 focus the cold body was situated, and the least when the metallic 

 surface was presented. Dr. Murray, alluding to M. Prevost's 

 theory, remarks, that " this explanation appears at first view, not 

 unsatisfactory, but it will be found deficient when applied to all the 

 phenomena, and in particular, appears inconsistent with the effects 

 of different surfaces, in radiating cold. The principle is, that the 

 cold surface is radiating caloric towards the thermometer, only in 

 a smaller quantity than the thermometer radiates to it. Of course, 

 of different surfaces, which at a given temperature, radiate different 

 quantities of caloric, that which radiates least must be least pow- 



have least effect in counteracting the reduction of its temperature ; 



