254 Influence of Colour on the 



fected by determining the time of cooling with the same coating 

 upon different cylinders. Thus, Nos. 1 and 2 having been 

 coated with carbonate of lead, and their times of cooling through 

 forty degrees having been ascertained, all the results with the 

 various other coatings applied to these cylinders were comparable. 

 The numbers thus obtained will not be strictly proportional to 

 the radiating power of the substance used, for the whole surface 

 of the cylinders, including the ends, was not coated, and the 

 contact of the air and its consequent circulation exert a most im- 

 portant influence on the rate of cooling. This latter element has 

 been shewn by the experiments of Petit and Dulong to be^inde- 

 pendent of the nature of the surface, and as the amount of un- 

 coated surface remains constant, the greater effect of radiation 

 will appear by the more rapid rate of cooling, and the less by 

 the less rapid rate. 



I proceed now to examine the degree of approximation which 

 may be expected from the results of the experiments. 



First, A comparison of different observations on the same day, 

 under the same circumstances of the cylinders, and nearly or 

 quite the same as to the temperature of the room, will shew how 

 far accuracy is possible under the most favourable suppositions. 

 The following table presents the results of this kind obtained 

 during the entire series of experiments, with the ratios of the 

 time of cooling :— 



In the following table, ten of the ratios are about 1.01 to 1, 

 six 1.02 to 1, three 1.03 to 1, one 1.04« to 1, and two 1.05 to 1. 

 It is therefore fair to infer, that the single ratio of 1.14 to 1 re- 

 suits from an error of record or observation, and the table fully 

 shews, that under the same circumstances the results could readily 

 he reproduced within about two per cent. 



I 



