Radiation of Non-luminous Heat. 251 



but by ascertaining for each subr.tance that thickness beyond 

 which radiation does not take place. This will be placed in a 

 clearer point of view in thp sequel. 



I do not, however, consider the question at issue as the less 

 difficult to determine, " no substance can be made to assume 

 different colours without at the same time changing its internal 

 structure ;" * and I believe, with Professor Powell, that " a very 

 extensive induction is perhaps the only means open to us of 

 ascertaining this (the circumstances and properties wherein the 

 coatings differ), considering how totally ignorant we are of the 

 peculiarities on which their colour depends.'" This very exten- 

 sive induction I do not pretend to have made, but I have multi- 

 plied our experiments so much beyond the number made by Dr 

 Stark as to be able to shew that the supposed influence of colour 

 on the absorption and radiation of heat remains yet to be demon- 

 strated, and thus to prevent the admission, as proved, of what 

 is more than doubtful. The principal object was to select a con- 

 siderable variety of pigments of the same colour, differing chemi- 

 cally, and of different colours chemically allied, and, as subsidi- 

 ary, to ascertain the effect of changes of colour produced by 

 chemical means on different substances, and the effect of the ma- 

 terial used to apply the pigment to the radiating body. Seve- 

 ral tin cylinders were procured, two inches high, and one and a 

 half in diameter, closed at the bottom, and having fitted to the 

 top a slightly conical tube to receive a perforated cork, through 

 which to pass the stem of a thermometer. One of these vessels 

 having been selected, was coated in successive layers with a pig- 

 ment. Water which was boiling in a porcelain capsule was then 

 poured into the cylinder, which was suspended by means of two 

 lateral hooks to cords attached to the canopy covering the lec- 

 ture table. A thermometer introduced through a cork had its 

 bulb nearly in the middle of the axis of the cylinder, and the 

 thermometer, by displacing part of the water, proved that the 

 quantity contained was the same in each case. A temperature 

 was selected for beginning the experiments sufficiently below 

 that which the introduction of boiling water produced, to per- 

 mit the rate of cooling to have become uniform ; and one for 



• Professor Leslie's Essay on Heat. 



