246 



Differentiating 



or 



HEAT. 



dt 



-Tt 

 dt 



that is, the rate of fall is proportional to the excess. 



If this law held good for any value of the excess the radiation curve 

 would be a straight line. But it is only true for small excesses, so small 



19 

 18 



drawn, through, observed points 



^^-'--^^ 



20 40 60 80 100 120 140 160 180 200 220 240 

 Excels of temperature of bulb over temperature of enclosure 



FIG. 141. Dulong and Petit'e Cooling in Vacuo 



that we may regard the radiation curve as straight, and consider that we 

 are moving along the tangent. It therefore gives us no information as to 

 the shape of the radiation curve. 



Dulong and Petit's Law Of Radiation. Very extensive researches 

 were made on the cooling of thermometers in enclosures by Dulong and 

 Petit about 1817.* Their chief work was carried out by suspending a 

 thermometer raised to any temperature up to 300 0. in a copper globe 

 surrounded by water-. The globe was then exhausted to a pressure of 2 

 or 3 mm. and the rate of cooling of the thermometer was observed. 

 Corrections were applied for the effect of the residual air and for other 

 disturbances, and the results were taken to give the cooling due to radia- 

 tion. They are represented in Fig. 141 by the curves I. to V., the tern- 



v 



* Ann. de Chemie, vii., 1817, or Thomson's Annals of Philosophy, xiii. , 1819. 



