RADIATION AND TEMPERATURE. 



245 



Various attempts have been made to find how the quantity R repre- 

 senting the radiation from a body at absolute temperature e, in an 

 enclosure at the absolute zero, depends upon the temperature. Since we 

 cannot have a zero enclosure it is obvious that we must deduce E, from 

 observations of the difference 



where 6 is the temperature of the enclosure or surroundings and e the 

 excess of the radiating body above that temperature. Let us suppose that 

 we have succeeded in determining the form of the curve representing the 

 radiation of a body at different temperatures in a zero enclosure, and that 

 it is OPQ in Fig. 140a. Let us draw the ordinate PM at and QN at + e 

 and PR parallel to the temperature axis. Then PQ, with axis PR, 



FIG. 1406. Curves of Cooling for Temperatures e, + e, 

 6 + 2e of the Surroundings. 



will represent the cooling curve in an enclosure at temperature 6, for 

 QR = R s+e - R. Suppose that we obtain successive cooling curves, say 

 first between Q for the enclosure and + e for the body, then for 6 + e for 

 the enclosure and 6 + 2e for the body, and so on. Let these curves be re- 

 presented on a single diagram, as on Fig. 1406, each curve being marked 

 with the temperature of the enclosure ; by putting these curves end to 

 end we can build them up into the radiation curve of Fig. 140a. 



Newton's Law Of Cooling. .Newton * made the first experiments 

 on rate of cooling, and found that the rate was proportional to the excess 

 of temperature above the enclosure. This may easily be verified by raising 

 a thermometer some 20 or 30 above the temperature of the room and 

 observing its fall. Plotting the logarithm of excess of temperature, against 

 the time, the result will be found to be a straight line. If e be the excess 

 at time t, and if E be the initial excess, the observations show that 



e 

 lo= -at 



* Phil. Trans., 1701, p. 828. Newton really investigated the convection effect 

 in a current of air. See Russell, Phil. Mag. (Q), xx. p. 599, 1910. 



