o 



20 On the Influence of Color on Radiation. 



jection to this method, and one which most affects the certainty of 



the results.* 



The following example shows the application of this method. The 



observed times of cooling of the standard cylinder, from 180 to 140 

 in two experiments on the 3 1st of October, were 969£ and 968 £ se- 

 conds, mean 969- Three experiments on the first of November, gave 

 898, 892, 8934 seconds, mean 8941. 



Cylinder, number four, coated with cochineal (crimson) gave for 

 the time of cooling from 180 to 140° on the 1st of November, 848 J. 

 To compare this with a result obtained with the same cylinder on 

 the 31st of October we have 894i : 969: ;848£ : x, the equivalent 

 number for October 31st, 916.3 seconds. 



The results obtained with the same cylinder on different occasions 

 of experiment, having been thus rendered comparable, the compari- 

 son of experiments with different cylinders, was effected by deter- 

 mining the time of cooling with the same coating upon different cyl- 

 inders. Thus, numbers one and two having been coated with car- 

 bonate of lead, and their times of cooling through forty degrees hav- 

 ing 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 important influence on 

 the rate of cooling. This latter element has been shown by the ex- 

 periments of Petit and Dulong, to be independent of the nature of 

 the surface, and as the amount of uncoated 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 un- 

 der the same circumstances of the cylinders, and nearly or quite the 

 same as to the temperature of the room, will show how far accuracy 

 is possible under the most favorable suppositions. The following 

 table presents the results of this kind obtained during the entire se- 

 ries of experiments, with the ratios of the times of cooling : > 



* If the circumstances could be retained Ihe same, three observations of the tern- 

 perature at equal known intervals, would give a numerical expression for the 

 radiating power of the coating. 



