RADIANT HEAT. 347 



experiments, I have expressed my objections to the inferences made 

 from them. 



It appears in general that the texture and nature of the surface most 

 unquestionably exert a great influence. Now, wherever, there is a 

 difference in the color, there must be either a difference in the mechani- 

 cal structure of the surface, or some new matter added or abstracted. 

 When, therefore, we consider the changes which thus occur, we can- 

 not infer that the effect is not owing to these instead of to color as 

 such. The question, however, is a highly curious one, and worthy 

 the most accurate investigation. 



Having in some measure called attention to it in my former report, 

 it was with no small gratification that I found the subject had excited 

 interest, not only in this country, but also in America ; and to Pro- 

 fessor Bache (since appointed principal of Girard College) we owe 

 by far the most extensive and valuable series of experiments on this 

 important but difficult point of inquiry ; they are given at length in 

 the Journal of the Franklin Institute, November, 1835. 



The notices of these experiments which had been published in this 

 country not appearing to convey adequate notions of their nature or 

 value, I endeavored to bring them more prominently forward by 

 some remarks in the Physical Section of the British Association at 

 Liverpool in 1837.* In my former report I had thrown out some 

 suggestions both as to the want of such a series of experiments, and 

 as to the fundamental difficulty arising from the variety of causes 

 which must influence the results ; but more especially the differences 

 of thickness in the coatings, which in the ordinary mode of operating 

 could not be estimated, yet must greatly modify the effects. 



With reference to the necessity of equalizing the coatings, Mr. 

 Bache refers to an important observation of Leslie, viz: that radia- 

 tion takes place not merely from the actual surface, but from a certain 

 depth, or lamina of the surface, the thickness of which is quite appre- 

 ciable in good radiators, and differs for different substances. 



Proceeding upon this fact, the author justly observes, that "the 

 radiating powers of substances would not be rightly compared by 

 equalizing their thicknesses upon a given surface, nor by equalizing 

 their weight ; but by ascertaining for each substance that thickness beyond 

 which radiation does not take place. ' ' 



It is, then, on the original application of this fundamental idea that 

 his whole series of experiments is conducted. 



Upon this principle the first object was to obtain some data as to 

 thicknesses of different pigments necessary to be employed. 



The method adopted throughout was to employ tin cylinders of the 

 same size, filled with hot water, and having thermometers inserted 

 through a hole in the top, while their surfaces were coated with the 

 different substances under trial. The radiation was estimated by the 

 observed rates of cooling. 



To find the critical thickness of the coating just spoken of, the time 

 of cooling a certain number of degrees was accurately observed, first 



° See Report, 1837. Sectional Proceedings, p. 20. 



