252 liifluence of Colour on the 



ending, which was high enough to prevent uncertainty from the 

 slowness of the fall of temperature. The instant of the arrival 

 of the mercurial column at any degree on the scale, and of its 

 leaving the same, was noted, and a mean taken for the time of 

 being at that temperature ; a precaution which, though superflu- 

 ous in such experiments as these, will, I am persuaded, be found 

 of importance where minute accuracy is desired in investigating 

 the motion of heat. One of us observed the thermometer, the 

 other noted the time by a pocket chronometer. 



The time of cooling of the cylinder coated with colouring 

 matter having been ascertained, an additional layer of the same 

 substance was put upon it, and the cooling again observed. The 

 time of cooling diminished of course until that thickness was at- 

 tained beneath which no radiation takes place, the time then 

 slowly increased with each additional coat, the conducting power 

 entering as an appreciable element into the rate of cooling. To 

 shew the decided nature of the results, I subjoin an account of 

 one series towards the beginning of our experiments, when a 

 want of experience rendered us cautious in applying the succes- 

 sive coatings, lest we should pass the thickness of determinate 

 radiation. The necessity for thus feeling our way rendered the 

 labour of the experiment very considerable. 



Cylinder coated with Prussian Blue. 

 Time of cooling from 180° to 140° Fah. 



1. Thick coating, 



2. Do. added, 



3. Additional coat, do., 



4. Do. do., 



5. Do. do., 



6. Do. do., 



10114 seconds. 

 965 

 910| 

 8291 

 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. 



1. First thick coating, .... 985 seconds. 



2. Additional coat, 855 



3. Do. do., 8274 



4. Do. do., 834i 



