348 



RADIANT HEAT. 



with a thin coating, then with an additional layer of the pigment, and 

 so on, until it was found that additional thickness did not increase 

 the rate of radiation, but began to diminish it ; thus each coating 

 was adjusted precisely to that thickness at which it produced its 

 maximum effect. 



Every precaution to insure accuracy appears to have been most 

 diligently taken, and several series of preliminary experiments are 

 recorded for the purpose of ascertaining the limits within which the 

 precision of the results may be relied on. A standard cylinder, coated 

 with aurum musivum (as being found not liable to tarnish or alteration,) 

 was used in all the experiments, and the effect of each coating com- 

 pared with this under similar circumstances. 



The results of different sets of experiments are given in the tabular 

 form, and apply to coatings of a great variety of substances differing 

 in their chemical nature, as well as in roughness, texture, and color. 

 The following table is extracted as fully exhibiting the general result 

 of all the experiments; the substances being arranged in the order of 

 their radiating powers, beginning with the JiigJiest : 



It thus distinctly appears that through so extensive and varied a 

 range of differences in the state of the radiating surface no determinate 

 relation subsists between the radiating poiver and either darkness of 

 color or any other distinctive character of the coating employed; not 

 even its roughness or smoothness. 



