On the InfMence of Color on Radiation. 19 



results, I subjoin an account of one series towards the beginning of our 

 experiments, when a want of experience rendered us cautious in ap- 

 plying the successive coatings, lest we should pass the thickness of de- 

 terminate radiation. The necessity for thus feeling our way, rendered 

 the labor of the experiments very considerable. 



Cylinder coated with Prussian blue : 

 Time of cooling from 180° to 140° Fah. 



101 H seconds. 

 . 965 

 . 910f 



. 805 



842 



Another series, in a further advanced stage of our experiments is 

 subjoined : 



Cylinder coated with Litmus blue. 

 Time of Cooling from 180° to 140° Fah. 



985 seconds. 

 . 855 

 827i- 

 . . . 834^ 

 Besides the necessity of making several experiments to obtain a 

 single result, it sometimes occured that particular results required to 

 be repeated for verification, when apparent discrepancies occurred ; 

 this was done to ascertain if they were real or not. 



As it was obvious that the experiments must necessarily extend 

 through a considerable time, during which the circumstances attend- 

 ing the cooling of the cylinders could not be expected to remain uni- 

 form, a standard for comparison was provided, in a cylinder of which 

 the coating was not changed, and which was observed in regular turn 

 with the other cylinders. At first a vessel without coating was used for 

 this purpose, but as it was found liable to tarnish, a cylinder was sub- 

 stituted having a coating of aurum musivum, which was one of the 

 smoothest and most uniform of the colored substances used. — The 

 numbers obtained on the different days from a mean of the trials made 

 of the cooling of the standard cylinder, were applied to compare the 

 the results of one day with those of another. This assumes that the 

 times of cooling of the different vessels would be affected proportion- 

 ately by a given change in the circumstances of the experiment. 

 This inability to preserve the circumstances constant is the real ob- 



