438 Mr. C. H. Lees on the Law of Cooling, and its bearing 



The constancy of the numbers in the columns of first differ- 



/ s/i n ~~ 1 \ 



ences (the columns headed 20 ) shows how well 



V mc J 



the assumption that the rate of loss of heat varies as the 

 nth. power of the temperature-excess can be made to represent 

 the actual fact by a proper choice of n. Within the limits of 

 the experiments, i. e. 80° to 10° C. temperature-excess, there is 



no definite secular change of the mean values of , and 



° mc Q 



we thence conclude that the above law is a close approxima- 

 tion to the actual fact. 



The values of h/c Q deduced from the experiments are, for 

 copper, iron, and zinc, respectively *0640, '0520, '0676, where 

 h is the amount of heat lost from 1 sq. centim. of the surface 

 in 1 minute, when the temperature-excess is 1° C* and tem- 

 perature of air is about 18° C, and c is the specific heat of 

 the material of the bar at 18° 0. 



Taking the specific heats at 18° C. as '092, '112, '0915 re- 

 spectively!, and dividing by 60, we have for the amount of 

 heat lost in 1 second under the above conditions : — 

 •0000981, -0000971, -000103. 



On account of the uncertainty of the specific-heat values, 

 and the fact that the zinc bar had not so good a surface as the 

 others, not much weight is to be attached to the differences 

 between these three results. The value deduced from the 

 copper bar is probably nearest the truth ; and we have then 

 the loss of heat in 1 second from 1 sq. centim. of surface of 

 the bars used equal to *000098(?; — V) 1 * 21 heat-units. 



The particular value 1*21 of the index n refers only to the 

 cooling in the water-jacket, but it seems not to depend to any 

 great extent on the presence or not of the jacket, for when the 

 cooling was performed in the middle of the room at a con- 

 siderable distance from any object the value of n which 

 best represented the cooling was less than 1*21 but greater 

 than 1-2. 



The value of n is, however, dependent on the nature of the 

 surface, and also on the cross section of the bar; for by 

 covering the iron bar with a shining black varnish n was 



* That is, if the loss of heat varies for temperatures below 10° C. ex- 

 cess, as it is found to do from. 80° to 10° C. excess. 

 t Naccari gives : — Cu '092 ) 



Fe -106 \ Beibl. xii. p. 326. 

 Zn -0915 ) 

 The values for Cu and Zn agree with the results of other experimenters, hut 

 that for Fe is too low. Brystrbrn's value = *112 is taken. Naccari's 

 values for c' have been used in each case. 



