68 Dr Stark 07i the Injluence of Colour on Heat. 



degrees ; and that while the black part of the dress is quite hot 

 to the touch, the white will be quite cool. " Again," he says, 

 ** try to fire paper with a burning-glass. If it be white you 

 will not easily burn it, but if you bring the focus to a black spot, 

 or upon letters written or printed, the paper will immediately be 

 on fire under the letters.*" He further remarks, that fullers and 

 dyers find black cloths, of equal thickness with white ones, and 

 hung out equally wet, dry in the sun much sooner than the 

 white, being more readily heated by the sun's rays ; and that 

 before a fire, heat penetrates black stockings sooner than white 

 ones. 



This sagacious observer now made an experiment with patches 

 of different coloured cloths from a tailor's pattern-book. These 

 were laid out on the surface of the snow in a morning of bright 

 sunshine. The colours used in the experiment were black, 

 deep hlue^ lighter blue, green, purple, red, yellow, white, and 

 other colours, or shades of colour. In a few hours the black 

 had sunk so much as to be out of the reach of the sun's rays ; 

 the dark blue almost as low ; the lighter blue riot quite so much as 

 the dark ; and the other shades of colour less as their tint was 

 lighter ; while the white cloth remained on the surface of the snow, 

 " not having entered it at all." * 



This experiment demonstrated, like all th^ previous ones, that 

 the calorific rays were absorbed much more abundantly by the 

 black than by any of the other colours ; that the intermediate 

 colours possessed a power of absorption in proportion as they 

 receded from the black colour ; and that the white portion of 

 cloth had reflected nearly the whole rays. 



An experiment similar to this, but with coloured metals, was 

 made by Sir Humphry Davy, and the result published in 1799. 

 This celebrated chemist took six pieces of copper (each an inch 

 square, and two lines thick), of equal weight and density, and 

 coloured one of their surfaces zahite, one yellow, one red, one 

 green, one blue, and one black. On the centre of the under 

 surfaces was placed a portion of a mixture of oil and wax, which 

 became fluid at 76°. The plates were then attached to a board 

 painted white, and the coloured surfaces of all the pieces equally 



• P'ranklin's Works, vol. ii. p. 100. Lond. 1816. 



