Dr Stark on the Influence of Colour on Heat. 69 



exposed to the direct rays of the sun. The result was, that the 

 cerate on the black plate first began to melt ; then that on the 

 Hue ; next the green and red ; and lastly, the yellow. The 

 square coated with white was scarcely affected by the heat, 

 though the hlacTc had completely melted. * 



The last of these experiments, though confirming as to co- 

 loured metals what Franklin had ascertained as to coloured 

 cloth, does not seem to have led to any further investigation as 

 to the modifying effect of colour on the absorption or emission 

 of heat. 



Though I have mentioned the experiment of Sir Humphry 

 Davy in connection with, because it seemed more illustrative of, 

 Dr Franklin's experiment, yet it is proper to remark, that there 

 were previous inquirers into the subject of heat and colour. 



The effect of a coating of black in raising the temperature of 

 substances induced Dr Watson, afterwards Bishop of Landaff, 

 to apply it to the thermometer in 1772. He exposed the bulb 

 of an excellent thermometer to the direct rays of the sun, when 

 the sky was perfectly free and clear. The mercury rose to 180° 

 Fahrenheit, and remained stationary. He then covered the bulb, 

 by means of a camel-hair pencil, with Indian ink. The mercury 

 sunk a few degrees during the application of the coating, and 

 the evaporation of the water ; but immediately afterwards rose to 

 118°, evincing a rise of ten degrees from the application of the 

 black coating. Dr Watson, though he does not seem to have 

 carried the investigation farther, concluded from this experi- 

 ment, that " if the bulbs of several thermometers were painted 

 of different colours, and exposed to the sun at the same time for 

 a given period, some conjectures respecting the disposition of the 

 several primary colours for receiving and retaining heat might 

 be formed, which could not fail of being, in some degree, inte- 

 resting t-'^ 



Count Rumford was the next writer of note who drew the 

 attention of scientific inquirers to the subject of heat. In a 

 paper read before the Royal Society in 1792, he detailed nume- 

 rous experiments on various substances to ascertain the cause 

 of the conducting and nonconducting powers of bodies with re- 



* Beddoes's Contributions to Physical Knowledge, pp. 44-5. Bristol 1799. 

 t Phil. Trans. Abridg. vol. viii. p. 371. 



